Let's correct the term.

You must NOT ACTIVATE ANY SORT OF MULTIPLAYER WHEN YOU'RE IN THE PLANE!

ok.. actually let me put it this way.

Planes have a lot of signal shielding around their equipement and such to protect against stray signals beaming around in the plane. But it's hard to test every single wireless device in the world to make sure that they don't interfere, hence the blanket banning on radio devices on flight.

The reason you have to shut down your things during take off and landing is that there are certain instruments that are used during those times which are extra sensitive to inteference and let's just say it's not a good idea to have an instrument go down when the ground is approaching at extremely fast speeds

I guess in summary is, yes it should be pretty safe to play wireless multiplayer when you're in the plane but if the people tell you to turn it off. Don't argue with them!