I think this is my favorite part of their FAQ:
Is running with the bulls dangerous?
Of course! Much like rock climbing, mountain biking, skydiving and other extreme sports, running with live bulls is an inherently dangerous activity (which is why it’s so thrilling). By participating in the run, you accept the risk that you might be trampled, gored, rammed or tossed in the air by a bull, or bumped, jostled, tripped or trampled by your fellow runners. We do what we can to minimize those risks by using less-aggressive bulls than those used in Spain and allowing runners to hide in nooks and climb over the track fence if necessary, but make no mistake: you could get seriously injured in this event. That’s why there’s a mandatory insurance fee during the registration process and medical staff on site.