I think the other thing to take into consideration when determining if parents are not controlling their kids enough, is that the parent might not know what their kid is doing is a problem. (Not the unattended screaming for hours, that one is pretty obvious).
The first time I flew with my kids, 1 of which is on the "active" side of the scale, I was guilty of this. He less than a year old at the time and was jolly jumpering on the seat - standing facing the back and bending and straightening his knees. He was having a great time, happy and laughing. I had no idea he was bouncing the tray table of the guy behind until a particularly big bounce spilled the guys coffee and he was pissed off. It sounded like he had been getting progressively more irritated as time went on, but didn't say anything until he was mad.
That was more than 20 years ago, and I still remember how embarrassed I was to have been inconsiderate, and to have my kids perceived as misbehaving.
I think lots of parents are in the same boat, especially with the seat kicking and the IFE pushing. They don't know it is bothering anyone, and people who are being bothered often don't feel comfortable saying anything, sometimes until it is too late.