Cheating. New technology = new forms of cheating; and if the league allowed everything on the sidelines, it'd be near impossible for them to keep up with.

Predicting plays isn't an issue. Predict all you like. Stealing plays (& the play calls), that's the real worry, as it probably matters most in this sport than any other. Stealing signs in baseball is the only comparison I can think of.

The WSJ article linked by breakall mentions it a little, but nowhere do I see the word 'cheat'. I doubt the league would address a question about it seriously anyway as that would imply they don't trust teams' staff (which they shouldn't).