I had a (should I say "funny"?) experience this passed summer with the local trail riding barn. They put people on horses and then tie them to the fence and just leave (till they are done with all people).
Well... I think it's very dangerous: say, if the bee stuns the horse in belly and horse will go crazy it'll easily buck the person off right on fence (read broken bones, if not neck). Well, I DO understand bee can be anywhere, but at least in open spot there is more area to fall AND horse has freedom to move, so at least it won't panic being stuck, and can bolt or jump rather than buck.
So is it a NORMAL practice in barns? I've been just in 2 trail riding barns, and never seen it before that last one.