There's nothing against the rules of having alliances and fighting multiple people at once. Its not done because its accepted that its not fun for the kingdom being gangbanged, so it doesn't occur much.

There's nothing against the rules to have someone rob someone else for 2 months straight and never stop. Its not done because its accepted that its not fun for the kingdom being robbed, so it doesn't occur much.

I see absolutely no problem with responding to the latter with the former.