During a formation lap, a driver unfortunate enough to be starting from the pit lane can conceivably gain an advantage by breaking the speed limit. For example, if things were very touch and go with getting the car to the end of the pit lane before the lights go out, the driver could just floor it.
I don't think this is what Schumacher did and I don't think he gained an advantage today, but for the reason's I've stated and to keep things black and white I think it is the right thing to do to penalise a driver who speeds in the pit lane during a formation lap.
Having said all this, I suspect the penalty for speeding in the pit lane during qualifying is a fine? If that's the case I would argue that this should be penalised with a grid penalty instead as well, because an extra second gained in the pit lane could make all the difference if a team have left it a little late to send the driver out.
In summary, in my opinion ANY speeding in the pit lane on Saturday or Sunday should be penalised with a grid drop or a drive through. My opinion counts for nothing of course, it's what the rules say that counts. But I think this is probably what the rules are intended to say, at least for race day.
EDIT: All of this is without even considering the safety implications, which, like
Fenderman says, is the whole point.