No NEC reason exists for restricting such placement. Common sense will tell you that it is quite possible that something plugged in and hidden from view could be an invitation for a fire. With fixed appliances or entertainment units, etc., you have some ready indication that something might be wrong, when the units don't work right. Yet, we cram furniture (beds and dressors) against outlet all the time, and when a plug is barely making contact due to getting knocked around by the shifting furniture or the bed being made up, we end up with premature failure of the outlet/plug, or start a fire. If the outlet is used for an electric blanket, or waterbed heater, we might not realize that there is a faulty connection.
Isn't this concern basically similar to restrictions against passing cords through ceiling tiles--that you can't see how well the cord's plug is making contact with the outlet?
