Is this just a spectrum thing? I have Aspergers and as far as I can remember, throughout my 30-odd years on the planet now, I have had people laugh in reaction to me getting angry with them. Occasionally, a couple of times, the greater was my anger the greater their laughter, which almost seems to break a natural law if you ask me!! !
Whenever I've been on the receiving end of someone's ire, I NEVER laughed - only once as a kid with my dad, I can't even remember what it was about, but he really lost it i.e. "DON'T LAUGH!! !! !" and threw me up against the wall, yelling in my face (he wasn't abusive otherwise; this was just an extreme). But anyways, I can't say I've seen other people laugh when people get angry with them either. Maybe there's something that appears "unnatural" to others (NTs of course) from our irate reaction or maybe it's just a culmination of them already building up humour from our lack of getting the gist of a situation or underlying meaning, so we explode at their insensitivity and ignorance, then they let out the laughter.
Still - shouldn't make their reaction any more socially acceptable than our unintentional slip-ups...