Proportionality, I would say, but I'm not from the US and don't know US (case) law so I might be way off here.

If a playwrite playing the lead character in their own production stands on stage and confesses to murder, surely that should be seen as part of the work and not a confession of the playwrite. When it comes to musicians doing the same thing, the US has a long history of selectively prosecuting black artists.

Producing art about a character committing a crime is different from admitting that you’d committed a crime. One is art, one is (hypothetically) an admission of guilt. That’s the difference.