After that Riis just let the two second strongest guys on the team fight it out
Not exactly. Here's the story as told by can't-remember-who-need-to-google-it-sometime-might-have-been-the-spanish-mechanic who was in the car:
Sastre wanted to attack on the Bonette when Frank was in yellow and everyone else was at their limit (Andy excluded). Riis said "ask Frank". Frank said no. Riis proceeded to yell at Frank to attack and that "If you want to win the Tour, it has to hurt and it has to hurt NOW". Frank said he couldn't do it. Andy tried to convince Frank to try.
Sastre, being Sastre, played nice and just sat there, wasting a great chance. Frank, being Frank, was thankful and honest and thanked him after the stage. Andy, being Andy, yelled at Sastre through the media about some perceived (read: made up) lack of teamwork.
A bit different from just letting them fight it out
Still, it's relative true if you're referring to the Alpe d'Huez.