I think you should say something. My theory on it is if the parents aren't going to disclipline their child, I will. If it's done appropriately.
I have a spec needs child and was waiting in Toys R Us for them to bring something out of the back. DS was about 4 at the time and this little girl was on one of those rides where you pay 50 cents and ride with Elmo in the boat or whatever for a few mins. He started to wander over there and I kept my eye on him. He was about 20 feet away from the ride when the girl's dad started to SCREAM at my son to get away-I immediately ran over there. He had just wanted to watch her was all-he wasn't even close to her or wasn't trying to get on the ride or anything at all-just standing there. As soon as he saw me approach, he calmed down and was using this real sweet voice to my son saying, "It's my little girl's turn right now, see, then when she's done you can ride blah blah blah."
I went beserk and told the jerk that my son has special needs and he was just watching the girl-"I heard you scream at my son and it was totally uncalled for!!!" etc. He walked away from me. Right then, my package came from the back so I grabbed it and followed the guy out of the store yelling at him about how rude he is and insensitive he was.
To look at my son you'd never know he's special needs, but good grief-who screams at kids like that??? I was PO'd and do admit, I lost control. But I'm very protective of my kids and especially my son who doesn't understand things like other kids.
If my son had done something wrong and I didn't get after him for it, I can understand a parent jumping in, but not the way he did-it still shocks me to remember how he screamed at my son like that.