Using someone elses setup would be just rude...I can't imagine too many photographers doing that to another photographer. People might not be aware though, so they may not be doing it maliciously, they just don't realise what is rude or not in that industry and things like that. Sometimes you can just politely tell them "no" and most people would just say "oh sorry" and put their camera away without issue!

I just don't want to see it get SO posessive that there are no opporunties to practice your own photography, or simply take candids and things like that.

That's what happens at the Royal....the OPs don't even TAKE ring photos but you still aren't allowed at all. It's a complete waste of an opportunity. But that's probably the organisers, not the photographers. It's a shame anyway.