Hard to imagine that there is a HS coach worth his salt that isn't aware of the Summer TB situation.
If he is aware and puts this kind of stipulation out there he is a lunatic. If he is unaware then he is a head in the sand lunatic.
With that said, I believe the proper ground is that he would like for players to be available as much as possible and be aware of big events and avoid as many scheduling conflicts as possible. He should talk to them about leadership and commitment to teammates etc. to keep them from blowing it off. If he's not careful he might find a few things out about the kids he is dealing with. To the extent players are unavailable then play younger or less experienced kids and see how they do. After all the summer should be a pure development period for a HS program.