Mechanical failure is part of the race. The commentator said it might have something to do with Schleck using the wrong gear, so it could even be his own fault.
In one of the earlier stages (3r stage, iirc), there was a big crash with some of the favorites of the TdF about to loose a lot of time. The peloton decided to wait for those riders and afterwards a lot of people critiqued that decision.
Staying on your bike, not getting a flat tire, avoiding mechanical problems, .. it's all part of the race. There is no general rule of etiquette that says other riders can't take advantage. It was up to Contador himself to decide, he chose to attack ..
The commentators mentioned that during some stage of the TdF in the 70's Merckx saw his rival (Ocana) fall and refused to take the yellow jersey. But then again Merckx is the greatest of all time and he knew he could win any way he wanted to .
Generally it's a faux-pas to attack if someone falls because a fan interfered or something, but personally I am annoyed if they wait in any other situation.