Looks like the people who lose out here will be either the stores (used games no longer sell for as much) or the people who sell their used games back to the stores. And seeing as the prices paid by stores for used games are already shockingly low, I doubt they could take another $10 off the offer price.
As a buyer, I should get the advantage of an extra $10 discount on the used game if I don't want multiplayer. And my choice is nice and easy - if the used game isn't at least $10 cheaper, I'm not buying it. Right now in the UK it's quite common for the used game to only be $3-$5 cheaper than new.