It's not okay, and you of course try to stop it. That's a given, and part of the reason why I don't need the aggrieved traveler's input on the matter during the flight. But a child's feet in a carseat are going to bump the seat in front of them from time to time, because that's where they hang. If he's straight-up kicking it, I'd remove him to calm him down and tell him that's not okay. But if he's bumping it unintentionally, no more than periodically, and you turn around and pop off after the third contact, as you claimed you would earlier, that chat isn't going to go well for you if you're impolite.
Gain some perspective. If a kid's acting up, the parent is having as worse time than you, believe me, but everyone has to make do with the constraints of coach air travel. Simply forgoing flying with your kids for years on end is not a realistic option.
Almost everyone understands this, as the one time I experienced an in-air meltdown, everyone around me could not have been more sympathetic and helpful. If a seat gets kicked, the most usual reaction is for the person to tell me not to worry about it when I apologize, or even to play peek-a-boo with my son. One guy handed me his iPad so my son could read a Pat the Bunny app he happened to have for his kids. Everyone gets it, usually.
I'd call your irritation and confrontation approach to be pretty unique.
ETA: I'm not drugging a 15 month old for a two-hour flight. If I'm flying to Hawaii, Benadryl it is. But not to Chicago.