In the UK in general, you run into the back of someone, it is your fault in the eyes of the insurers. If you hit someone due to them brake checking, you were driving too close.

This is my internal logic too. Although upon some quick Googling I was surprised to find that in the U.S. it's a legal gray-area which varies from state to state.

How could they prove you were break checking? It seems like it would be easy to lie and say you thought you saw something running across the road and hit the brakes just in case it was a kid or something. Plus tailgating is also illegal, which their dashcam footage would prove

Which means both of you would get a ticket/citation/fine/etc instead of just one of you.

Brake checking was a popular thing to do in my state before dashcams were so popular and affordable (assumes the person behind is at fault, but brake checking is also illegal). Someone with money is clearly able to afford dashcams, hence, don't brake check rich people thinking "they have more to lose than I do" because you just think it's only your car that's at stake.

I do it out of prudence. The routes I take going to-and-from work often require sudden stops as you hit traffic bottlenecks. I'd rather force them to back up now while I have room to avoid an actual collision, than be rear-ended later when I have to brake.

I drive a stick - I can slow down fast without touching my brakes just by changing to the wrong gear. I've done this by mistake more than once - I'm not enough of a jerk to do this intentionally but it is a real situation that drivers behind me better be ready for.