From the way Cyclops spoke, it seems like it was meant to be a friendly gesture, to treat them as guests. Despite the teams' differences, they're all friends and want to be cordial, especially at a time like this.
I don't know why they didn't simply just avoid the Arbor Magna though. Or why they had to disguise it so; they could have just said it was a healing area or something, instead of explaining what actually goes on there.
The mutants had motive & opportunity to murder Wanda in secret and cover it up, but they're went out of their way to explain the situation and cooperate with the Avengers. They've also made it clear that they're investigating things on their end, and that the main suspect is Magneto. They're grieving and in a tense situation - they can ask about the suspects or look into things a little later, when they have Wanda's body.