its legal in the sense of...there isn't much you can do about it. most of the dm team won't like seeing this kind of action...but its well within someone's right to turn on their faction.
and as of right now there is nothing in place to stop them from being bound to their home or stop them from spam raiding. its been that way for a long time, but i would suggest that if your doing it....watch your back because dm's like to see a fair fight