I've never seen anyone use those sorts of locations to relieve themselves, but maybe I play with a different class of public urinator. There's a line for socially acceptable peeing behavior, and I think what you describe falls below it. That said, I suppose the answer to your question "why not wait?" is simple -- because we don't have to (neither anatomically nor socially, I suppose). In fact, I would bet that, if it were as easy and sanitary for women to take care of that function in the great outdoors as it is for men, women would not wait either (especially since the women I play with are constantly complaining about having to go).