It's not that the retailers are trying to lock them down, they'll happily sell DRM free books. However, publishers in particular tend to be sticklers for DRM, and won't sell on your platform if you don't implement it as an option.

There are books I'd buy today if they weren't published by publishers which only distribute with DRM. If it isn't desirable enough in that case to justify buying a dead tree version, I just pass entirely.