Why are there 'rules' in the OTAs ?

I read about teams getting fined for violating some rules during the OTAs. To me, coming from soccer background, it is sooo counterintuitive that some body is regulating how a coach is conducting his practice sessions.

What sort of restrictions are there generally and why? Are they primarily for players' safety? Surely no coach would like to get his players injured in practice so they wouldn't do anything dangerous, right?