I knew a guy that volunteered for the Student Conservation Association (the other SCA without all the fake chain mail) that worked at Yellowstone for a summer. He witnessed a man try to put his young son on the back of a Bison for a picture. The kid was thrown, but managed to avoid major injury - but the dad got gored. I don't know if he survived, but it just illustrates how stupid people are, and how signs like these should really be taken seriously.
I saw some of these magnificent animals long ago in Colorado, though they were safely behind a fence. It sounds like people behave as stupidly around these dangerous animals as they do around bears in a national park. No bovine should be trusted, whether it's a domesticated bull or a 2,000 pound bison!