Why no criminal charges for Weinstein?
Best chance would have been the time he got wiretapped, but the New York DA didn't pursue it (probably paid off). Most of the other instances are he said/she said without any physical evidence, and you'd more than likely run into the same issues they ran into when they tried to charge/try Cosby for what he did (statute of limitations, many of the other stories not being admissible in court for whatever specific instance they choose).
Edit: Also, as to why he can have this in his contract, IIRC TWC is what they started up after they left Mirimax, so he was basically already a huge name in the industry with a history of paying off women when the contract was drawn up. He basically had all the leverage and that type of language shows it. Also, yeah, people are going to start asking soon why Disney wasn't asking any/more questions while they were working at Mirimax.