I'd say including Casper is pretty unfair. Jimmy Casper's positive was for corticosteroids which he got a TUE backdated for. This was within the levels that he'd had a TUE for twelve years for, but due to an administrative mix-up the papers hadn't been renewed in time, and he was exonerated. A few of the others who have only sort-of been busted are mentioned above. However, no mention of Clément L'Hotellerie? Wasn't he straight up EPO? That'd suggest he was a more blatant doper than most of the above. Larpe was EPO as well iirc.
But of the upper echelon riders that fans trust at least to some extent, there is a disproportionate number of French riders. It is also, however, true, that they are mostly of a generation that is now aging - Bassons (1974), Moncoutié (1975), Casar (1979), Fedrigo (1978), Engoulvent (1979) - while obviously Bassons is the ultimate example due to his premature retirement from being hounded out of the péloton, there you have a few riders from the "péloton à deux vitesses" era who have carved out successful niches for themselves whilst being seen as mainly clean, either by being explicitly named as so, speaking up against it or generally being well-reputed. It is perhaps also worth noting that there aren't so many young French riders who either come with the cached goodwill of a Fedrigo or Casar or who are as willing to talk smack about dopers and ex-dopers as Engoulvent, and perhaps that does speak of the generational gap talked about above, with the "péloton à deux vitesses" era now being over.
I know there are also French riders from the generation born in the mid- to late-seventies who don't come with a positive reputation too - e.g. Gadret - but all of the French riders with the biggest level of trust, and among whom the stereotypical "French guys don't dope" reputation has been built, are of that older generation now.