I've unlocked all but two of the DLC guns and I haven't paid a dime. I think people's main issue with the system seems to be the random nature of it. The monetary angle is something that people like to complain about but like I said, the only people losing out are the people who pay; they're not guaranteed a weapon and even if they unlock one, they've paid actual money for a sub-par gimmicky gun or reskinned combat knife. Big whoop.
As it stands, supply drops are, for most people, an insentive to keep playing the game, and for others, something to spend their pocket money on. For Activision, it's a way to make an easy profit. Is that a bad thing? Unless they started hiding content behind a paywall, not really. Say what you like about the random nature of the supply drop system itself, but the problem certainly does not lie in the entirely optional monetary side of things.
I feel like this is probably a good place to point out that the inclusion of the Peacekeeper in DLC1 for BOII wasn't recieved with nearly as much flak as supply drops, despite the fact that you had to pay for that. Also, Battlefield has been doing about 5 DLC weapons with each map pack, which you not only have to pay for but unlock, and that system has also recieved nowhere near the amount of criticism. I suspect less people would be complaining if they just let you unlock BOIII's DLC weapons through a completely transparent monetary transaction.