The difference is that the rational response to "Fire!" is to get the hell out of there as fast as possible, which could lead to injuries etc. (Let's not derail into a prisoner's dilemma discussion about how the "real" rational response is for everyone to make an orderly exit). Anyone who partook in a panicked escape from a purported fire would not be held responsible for incidentally trampling other people who were also trying to escape. On the other hand, the rational response to hearing someone "incite racial hatred" is not to go out and start committing crimes against people belonging to that group.