First off, please do not offer subjective impressions of how a rider appears in a race as evidence or non-evidence of doping. Evidence of doping ought to be based on times, doping tests, evidence written and circumstantial, whistleblower testimony, etc.
Also, if Sastre and Nibali make vague anti-doping comments from time to time, this don't indicate anything about whether they are clean or not. Those of us who have followed cycling for a long time are aware that many cyclists who loudly claim to be clean have turned out to be dopers. The "he wouldn't claim to be innocent if he wasn't innocent" argument has no credibility after Hamilton and Landis, sorry.
More to the point, the fact that characters like Sastre and Nibali speak so little about doping as cheating should tell us something. If it were true that they are clean and their competitors were doping, then it would only be natural to say something about it--to call for better enforcement, clean sponsors, rail against institutional corruption, etc. Instead, they join teams led by prominent doper DSes that are closed affiliated to the likes of Fuentes. I've read that Sastre was angry about the way Jimenez died, but that doesn't mean he rejected the reality of the sport or what it took to win races. It's makes absolutely zero sense that a non-doping rider would join a team led by a Bjarn Riis (Sastre) or an Andy Riis (Evans).
The reason these riders don't speak is because they are loyal to omerta, the code of silence in cycling. The continued allegiance to omerta in the peloton tells us that cyclists are not free to speak their minds about doping, and so we have no idea what they truly think about the subject.
Let's focus on Sastre.. throughout his career, Sastre has been part of doping teams. In every race he's won, he's defeated known dopers. The science of doping indicates that an advanced oxygen-doping program increases performances by 5-10%. So to believe Sastre is clean, you have to believe he has the superhuman power of defeating enhanced athletes, and that he's so talented that competitors who cheat to get that extra 10% still can't beat Sastre.
The first time I remember seeing Sastre he attacked the US Postal train in a super-steep section of a 2001 stage---Lance attacked a few minutes later and won the stage, but Sastre's attack impressed everyone watching. It was especially interesting given that Jalabert was (most likely with the help of PEDs) dominating a bunch of stages already for CSC. Like Jalabert, Sastre was a veteran of Manolo Saiz's ONCE team and had once been a much loved young rider under Saiz.
Then in 2003 Sastre won the stage to the Ax 3 Domaines. I guess he was clean in that too, and just got lucky. Stage results from the day...
1 Carlos Sastre (Spa) Team CSC 5.16.08 (37.48 km/h)
2 Jan Ullrich (Ger) Team Bianchi 1.01
3 Haimar Zubeldia (Spa) Euskaltel-Euskadi 1.03
4 Lance Armstrong (USA) US Postal-Berry Floor 1.08
5 Alexandre Vinokourov (Kaz) Team Telekom 1.18
6 Ivan Basso (Ita) Fassa Bortolo 1.20
7 Juan Miguel Mercado (Spa) iBanesto.com 1.24
8 Iban Mayo (Spa) Euskaltel-Euskadi 1.59
9 Christophe Moreau (Fra) Credit Agricole 2.32
10 Tyler Hamilton (USA) Team CSC 2.34
Then in 2006 (after providing valuable assistance to Basso in the 2006 Giro) leadership fell to Sastre in the Tour. Again, according to your perspective, all the bad dopers must have been ejected from the race, since Sastre finished 4th overall and was considered the strongest climber. He outclimbed his rivals on the Joux-Plane.
And of course, by that time he'd already placed 2nd in the Vuelta. Just for a bit more perspective, let's have a look at those standings.
**1 Roberto Heras Hernandez (Spa) Liberty Seguros-Würth Team 82.22.55 (40.74 km/h)
2 Denis Menchov (Rus) Rabobank 4.36
3 Carlos Sastre (Spa) Team CSC 4.54
4 Francisco Mancebo (Spa) Illes Balears-Caisse d'Epargne 5.58
5 Carlos Garcia Quesada (Spa) Comunidad Valenciana 8.06
6 Ruben Plaza Molina (Spa) Comunidad Valenciana 11.36
7 Oscar Sevilla (Spa) T-Mobile Team 13.22
8 Tom Danielson (USA) Discovery Channel 16.38
9 Mauricio Alberto Ardila Cano (Col) Davitamon - Lotto 18.15
10 Juan Miguel Mercado (Spa) Quick Step - Innergetic 18.31
So in this race, only Heras (Fuentes client who tested positive in this race) and Menchov (Humanplasma) beat him, while Sastre defeated Mancebo, Sevilla, Plaza, and Quesada (all allegedly Fuentes clients). See the list of riders implicated here
http://www.podiumcafe.com/2006/6/30/75424/2504
That's only looking at 2006 and before, yet as we know Sastre's most illustrious performances come later.
Based on the above, let's arrive at some conclusions....
1) If he was clean, then Sastre is clearly way more naturally gifted than the likes of Heras or Mancebo, who need oxygen-vector doping to beat or compete with Sastre.
2) If he was clean, Sastre is also an extraordinarily soft-spoken and philosophical individual, since he has finished 2nd and 3rd in so many Vueltas, and has lost to a number of confirmed cheaters, and has never once complained.
3) Therefore, if Sastre was clean, he is not human but a GOD.
But returning to the land of reality, of course there is no reason whatsoever to believe Sastre is any more clean than his colleagues.
You admit that if Basso hadn't been caught in Opertation Puerto, then you would give him the same credence you give Sastre. So essentially you are saying Sastre could be doping. Indeed, you seem prepared to acknowledge someone in Sastre's position has every reason to dope. You say we shouldn't condemn the dopers we "can't know about" but we should condemn the ones we do know about. This sort of reasoning presupposes a faith in the testing authorities that isn't warranted--keep in mind the UCI is led by the same people that Armstrong allegedly bribed and who have continued to try to distort the truth about doping in cycling at every opportunity. Puerto demonstrated the testing wasn't working and that Verbruggen and McQuaid had been feeding us lies. Yet just like after Festina, no leadership change.
There is no direct evidence to implicate likable Carlos Sastre, but the statistics and circumstantial evidence, combined with the reality that PEDs help improve athletic performances, do not point to a clean rider. The guy has ridden for dirty teams and dirty DSes his entire career and has never given the public a good reason to believe he's some kind of exceptional talent. If Sastre wanted support as a clean rider--all he has to do is ask for it and prove to our satisfaction he is clean. No rider or team in the current peloton is prepared to do this. So given the reality of the peloton (continued omerta on doping) I'm not going to extend the benefit of the doubt without justification, particularly to a rider that would have to be superhuman to have achieved what he has achieved sans doping.
Sastre can continue to ride and win races for as long as he doesn't get caught. He doesn't need my faith, and chances are he doesn't want it either.