They don't. And I wondered about this myself because I've never seen another team take over a stadium like we normally do. Saw the answer on another forum, where the member attended the game. They treated this game as a Vikings home game. And as such, they gave out purple flags for every seat in the house. So, anyone who wasn't a particular fan of any team, by default was waving purple stuff.
Yeah, the Vikes lost out on one of their home games to come across the pond. It makes sense that they would want to give them a bit of that "home field advantage."