It’s just because they feel awkward, for the most part. Even I laugh sometimes when it happens to me at a particularly inopportune time (which is often enough). I think there is a clear line, for me personally, between the type of stutter that makes people laugh and the type that makes them go “My God, What is WRONG with you?” or “Are you okay?!”. As a child, the later was more common but now the former is most common for me. I find that getting flustered by people laughing just makes it worse so I try to not be bothered by it nowadays. I must admit that I find this difficult because I am a very sensitive person regarding this. As weak natured as this might make me, I try to just react in a “haha, it sure is funny” type of way, rather than being openly dismayed by being laughed at, because sometimes people will feel less disturbed by me if they take it as not so serious. It’s hard to get anything done when the awkwardness of the initial reaction sours things. Some people will feel guilty but some people double down on treating you harshly. I like to just imagine that they don’t understand and so react inappropriately. Even I am guilty of laughing when I feel embarrassed or awkward and so I try to interpret it like that.