From a 400 person perspective, enforcing a "Semi Auto Only, Full Auto LMG" rule is easier than pulling players off who you think seem to be reloading not as often as they should be.
That isn't the primary benefit though as the semi-auto restriction has a fringe perk of encouraging squad tactics - if you want to lay down effective suppression fire you need to place that LMG guy carefully and work as a squad effectively.
It has it's ups and downs. I was using a WE GBBR at Nightfall 2 with 150 rounds total, and at a game with no ammo limits I was still managing to kill a lot of players, taking positions, and flanking the enemy with effective squad tactics.
Let them carry as many BB's as they want - didn't stop me last year.