I should think thats it's to ensure (esp. on band 1 flights) that there is sufficient time for pax to eat, and then for CC to tidy up again, before the seatbelt seats are switched back on. You can keep drinks after the signs have been switched on, but not food. The argument for this is not as strong for band 2s, but maybe it's to ensure the rules are less complicated.

I may be (and probably am) completely wrong.