It sounds to me as if you take things too personally. The problem that I have with kids throwing tantrums isn't with the kid. They are often tired, hot, over-stimulated, and excited. Kids are going to be a bit hard to handle if you don't have children yourself.
The problem is with parents. Let me put that in caps. THE PROBLEM IS WITH PARENTS. When I see a kid acting like a complete pain in the a$$ and the parent isn't doing ANYTHING, or is trying to bribe their child to act like a human being, I'm going to roll my f'in eyes. You are damn right.
But I'm sure that you, being the parent of an autistic child understand that you can't just sit back and say "honey, what is your problem? would you like to talk about it?" or "just let him scream himself out".
If you don't want rolled eyes or comments, then say "hey, you try raising an autistic child". But to condemn everyone that looks at a screaming child with disdain is just as narrow minded. You are in the minority, don't expect everyone to walk on egg shells just in case they come across some kid acting like a brat because they are autistic. Be pro-active, and don't be give the impression that you are enabling your child.