http://fallout.wikia.com/wiki/PIP-Pad
It's a PIP device in their Fallout Online MMORPG. I mean, sure they're allowed to make their Fallout game, but it's canon for the rest of the games that they're messing with here. It has a whole canon backstory and everything. So, can they do this? They're messing with the storyline of the Bethesda owned Fallout Franchise.
Please note this isn't to discuss the legalities of the dispute, but whether or not Interplay is/should be allowed to add in their own stories that can affect the other storylines.