I disagree only in that it should not have to be made mandatory or put in the contract. The players are supposed to be professionals and as such should be motivated enough, and professional enough, to keep themselves in shape and not show up to camp as an overweight out of shape slob.
My daughter is a high school basketball player. During the entire off season she is working with a coach, and doing strength training etc. to keep herself fit and strong and improve her game. She does this willingly on her own. You would think all professional athletes would care at least as much as a high school kid.
But, some people simply have no self discipline and cannot workout without someone forcing them to. Multimillionaire athletes however, can and should hire someone if they lack the self discipline themselves. Were I a NFL GM I would have absolutely no patience for anyone who disrespected the team enough to show up to camp out of shape.