On Southwest, you can choose your own seat given your place in line. We boarded in the A group and she sat down after we already boarded.
FAA regulations state that a child under 2 must be on a parent's lap or in a car seat in a window seat. My 15 month old was in a car seat in a window seat. We couldn't move him anywhere else. Putting a car seat in an airplane is a pain and a half. Obviously, it would have been ridiculous to move a 15 month old and a car seat in the middle of a flight and that would have bothered many more people.
By the way, she was a complete bitch and personally insulted my son saying, "I'd hate to see what he's like when he's older".
Maybe I'm unique, but I've never been bothered by kids on planes even before I became a dad. If parents didn't fly with their kids, ticket prices would be higher. Having a child is stressful and flying with one is even worse. Being a dick to a parent just makes it worse for them.