If you hear a rumor that a bad thing happened, you can't speak about it too much because you weren't there and can't provide eyewitness testimony.  You also can't say where you first heard it, because it's at best second hand and in Weinstein's case the victims were bound by strong confidentiality agreements.

You also can't file a police report, or run down to his office and throw some handcuffs on him.

A joke is the only way you can bring the rumor to light and create public awareness.  With Cosby, there was a joke on 40 Rock that started the ball rolling on changing public perception and getting people talking about it.  With Weinstein there was this joke at the Oscars and another joke on 40 Rock.