I'm sure there are weapons in the game that don't use magazines that still have fast mags available as an option.
They all use mags, but with the Remington an KSG, you just load each shell quicker.
because you can't run around like a chicken with its head cut off when you have a LMG, just use the miniature LMG called the PDW does more damage anyways
awensering to the original question... i think they done it because the massive firepower you can unload with a LMG, sometimes you can lock down an entire area if you use it correctly, and if it is hard to flank you, opposition would be quite screwed, the only chance they have is the time you take to reload your gun.
in simple words, i think that it is so you cannot abuse of a constant stream of bullets.
excuse me but the post is about the LMG's the PDW is a SMG with a big magazine, it has its role in the game and surely its not the same role of a LMG.
So i dont' see the connection.
Because most LMGs are belt fed so it couldn't logically have a fast mag and the ones with clips have LARGE magazines. I honestly don't see the problem here, you get alot of ammo anyway. Just be smart about when you reload.