This is also called "common courtesy". And it's also the reason that I don't ride at schooling or novice shows, because there is none.
As someone who doesn't just ride around in circles, it is very difficult to school my horse around people that don't know what the heck courtesy is! I am very courteous to less advanced riders and call my lines, but still some people cut me off. I am so tempted to run them over sometimes.
Also - why I hate jumping shows!! In the warmup, people call their fences like 3 strides out.. -_- So not cool I have almost been killed by idiots calling their fences when they are halfway over them. And at a lot of jumping shows, they do have the everyone goes in the same direction rule. Because there are 40 horses, and fences, and people jumping fences in a 80ft x 200ft ring.