No bell boots, no boots, no polos. You aren't allowed any leg wear at all regardless of the foodting or ground you are showing on. Once you head into jumper classes, your possibilities are practically unlimited, but as far as legwear in hunters, nope-nothing.
When you think about it, you shouldn't need anything. The space in the arena, the distances and the way the fences are build are all user friendly. They are made to allow you take the safest jumping course you possibly can since it's based on rider and horse movement, control and riding ability.
When in Jumpters, you are expected to change your paces throughout your course. You are expected to be cutting corners, changing your stride in between fences. You are expected to do rollbacks etc etc...
I myself would prefer it, if they allowed protective gear regardless of what you are doing but...it isn't so.