"The first one is the "He behaved bad in the past so he won't behave now" thought, and you just can't know that."

Right, you can't know that. You can only project future behavior on the basis of past behavior. And just perhaps, the rights of the award nominees not to worry about having their Big Night tarnished by some MC thinking it's funny to crap all over them is worth equal consideration to JR's feelings. Or was this not about the Hugo award nominees at all? Maybe I missed the part where the award ceremony doesn't have anything to do with them.