I think there are several factors. First, it's kind of an invisible/stealth disability. You walk into a room of people; you'd never be able to pick out the stutterer. No white cane or seeing eye dog. Then there is the fact that stutterers often try to hide it. Feel ashamed of it. That might feed the mockery. Also, consider that everyone stumbles or stutters to some degree. Fluency is a spectrum, like anything else. So when some people mock others, they may not realize the gravity of the situation. But I really want to emphasize that it is NOT OK TO MOCK OTHERS. I don't want to be mistaken as excusing bad behavior. Just trying to answer the question posed. Stutterers should not tolerate such mocking. Call people out on their BS. I realize this is tough for a stutterer to do. But to allow the mocking is worse.