Perhaps for the same reasons certain people are attracted to service in the military. Some people report getting a massive kick out of participating in active armed combat situations, to the point that civilian life seems dull in comparison.
You get a comparable kick playing paintball (not woodsball) or laser tag without killing anyone.
Not sure. The knowledge of fake risk ("you're being shot at, yes, but it's just paint-filled balls - painful yet nonlethal") diminishes the fight-or-flight reaction.