Guess what... You wonce were the screaming kid.
But seriously, I have two children, and while I never tolerated bad behavior and tantrums, well, you can't always control them, and they have to learn. It's not an instant process. Meanwhile, I don't think it's reasonable to ask families to abstain from public appearances. And it's not always possible to do the shopping without the kids. And even if it were, how would they learn how to behave in public if you never expose them?
I sympathize for the inconvenient, and I'll admit that some parents are careless about this... But it's part of life in society.