Why is it that it's okay to make fun of a stutter?

Why is it that it's okay to make fun of a stutter? Getting a cheap laugh from that socially awkward dude that could barely finish a sentence just now? Haha! Absolutely hilarious! Let's also mimic his stutter so he's aware of how dumb he sounds! Make fun of someone else's sexuality, religion or even a disability ? Everyone is mortified! How dare you hurt their feelings like that?