The only time I have seen this rule actually enforced was on a flight from SEA-MSP in the J cabin; someone in front of me kept using their 15" laptop while we were on the ground to look at the results from their dating service. When asked to put it away for departure, they'd stow it in the seatback and then immediately pull it out again when the FA turned their back. Unsurprisingly they were forced to actually put it away and were told this wasn't permitted in the seatback.