As someone who recently was the parent of one of those screaming kids, I'd note a couple of things:
1. No matter how bad it is for you to listen to the kid screaming, it's infinitely worse for the parents, who are both frustrated and embarrassed. I travel a lot (about 60K miles per year) and understand what it's like to be subjected to kids crying on a flight, especially when you're also tired and ready to get home.
2. Sometimes, no matter what you do as a parent, the kid is going to scream. My son just turned two years old. The flight was delayed 3 1/2 hours (try getting a kid to sleep in a busy terminal) so we took off about the time we should have been home. The people in front of us put their seats all the way back so that there was no room for me to hold my son, making it difficult to console him. And hitting a 2-year old in pubic in this day and age is generally frowned upon.
So not to make excuses, but it's not always easy to quiet a young child. Trust me - we tried everything.