No there's no safety angle to it. As with all carriers the bag must be fit under the seat and not cause obstruction and the only exception of course is at the exits.
With the odd exception who gets past the gate staff, people don't over pack if they are within the dimensions set out by the airline. As crew we ask for consideration by allowing pax with roller bags and large holdalls to place them in the locker ( oh and please place them side ways ). I'm usually at the front being a CM but occasionally like to stand in the cabin for boarding and I don't mind admitting that on busy flights I apply the policy strictly. I take out small items and politely ask pax to place under the seat... At least until boarding s complete then they can place the items in the overhead provided there is space.
But yes to answer your question it is policy and several months ago boarding PA's were adapted to include it. In any case many of the smaller items don't impede on legroom. I.e carrier bags, hand bags and even rucksacks.