I hate when parents use oh my child has autism as an excuse when their kids act up
Why do you hate that?
They should be bringing it up in private and not in public for people to know.
Most people shouldn't use autism as an excuse for their kids acting up. I can understand why you would be upset, because even many kids with autism can be taught.  But there are times when children simply don't know how to behave in public.
Autism is not an excuse it a reason.
You're right that it shouldn't be a catch-all excuse, however severe cases of autism could indeed be the reason why children are acting up in public.  Since it doesn't really cause you any problems, it is best to just be tolerant of the child's behavior at this time.  It is obviously much harder for the parents, so you should cut them some slack.