Not to be a curmudgeon about it, but they see it as if you, knowing the person is in front of you buying a gun, is going to shoot someone with it, would you still sell them the gun?
I'm sure someone would, somewhere, but I'm not going to sell that gun to that person, knowing there's going to be death because of it. Not that I personally draw the line of life or death at 16 cells getting ready to turn into 32, or anything, but that's their viewpoint and they're welcome to it, imo...
And yes, those service people (the privately employed ones anyways, the bus driver and mail person are different d/t employed by our government) the carpenter, painter, plumbers can and do refuse working for certain people, whoever they may be, all the time. As is their right, IMO. What I find distasteful, I can steer clear of.