I don't know about magazines, but I will tell you that there is a new FAR which says that heavy objects, such as personal computers, may not be placed in the seat back pocket for taxi, takeoff, or landing. These rules usually arise after something has come out of the pocket, gone flying across the cabin, and injured someone.
As far as I know, magazines are ok, but have any of you seen this magazine called, In Style? It and Vanity Fair must weigh 10 lbs apiece! They should be secured in the o/h bin or under the seat also.