What if it turns out Wanda isn't a mutant (like they've been hinting at in Uncanny Avengers)? What should stop the X-Men from revealing the truth then?
Unless you were possessed by the Phoenix.Once the issue of possession was brought up, the Avengers and X-Men again jointly agreed she shouldn't be locked up. Every hero at one point or another has been mind controlled or possessed. It's always been the comic book free pass.