I believe it is a true rule as I've had many airlines (DL, AA, US, UA) announce it. Some FAs are much more intense about it, however. On a recent AA flight someone did put their laptop in front of them and it fell out upon landing.. hurting no one other than the user and the laptop itself. Of course, that person was kind of an idiot and I think its perfectly save if you're not a moron to put the laptop all the way in, but rules are written to save morons from themselves (This was in first class, so stupidity knows no boundaries)