It's so obvious, isn't it?
More than 3 gathered together at the back of the plane are planning to rush the front of the plane. If it's 3 or less, they couldn't possibly do that.
AND, the planning can only occur while standing in line waiting for the toilet. Couldn't do it sitting down in their seats, 'coz terrorists can't think while sitting down.
Harrass the general populace to give them the semblance of security!!!