Oh, I don't often play Cryro, and ... errr ... I wasn't thinking of becoming a traitor myself.
By the way, I can think of no way to implement this that wouldn't be completely open to abuse. Just throwing it out there as a point of exasperation, this evening. It doesn't happen often. Usually, when factions cooperate, it involves duelling or maybe PvEing together, but this was so underhanded. The guy didn't even participate in Molag Bal. His express purpose seemed to be standing guard against his own faction.
Actually there is something ZOS could do: Give me the option to crouch or cloak away from my own faction so that, like enemies, they can't see me either. Hey, since we're already getting rid of friendly combat visuals, maybe this will even improve performance further.