okay who many times have you had a marine frag you with no reason.... well I can't count that high but this is what happends when it happends every day.
Luckily, I haven't managed to get fragged by any of the marine 'ally's'... yet. However, I would recommend not giving any marines a fuel rod gun, or a M319 Grenade Launcher in Reach. I found those out the hard way.