I have every right in the world to attach stipulations to the sale of a horse, or a cart, or an axe if I wish. I can even ask you to make it all official-like by signing a legally binding contract with me, guaranteeing, for instance, that the horse will not be used for farm labor, and will be guaranteed at least 4 carrots a day with his feed, because he loves those carrots. I can't force you to accept these conditions post-sale, certainly, but before the sale is completed? I'm allowed to ask for just about anything I want, and ask you to sign a contract agreeing to it.
You are, as well, free to decline to enter into that contract with me. You may be looking to buy a horse to hook to a plow for plowing the fields of your farm, so the conditions I've attached to the sale aren't acceptable to you. Or maybe you're allergic to carrots, and having them in the house would pose a risk to you or your family. In which case you are *free* to refuse the contract as written, and negotiate with me for different terms ("I'll have the neighbors give him carrots once a week, on the day that I'm out of town. And I'll only use him to plow 2 small fields."), or simply seek to buy a horse from a different seller.
What you are not free to do is roll up in the middle of the night with a horse trailer and take the horse. And yes, the analogy is imperfect, because we're talking copying, not exchange of physical goods - but I'm not the one who brought up physical goods in a discussion about copyrights.
However, the principle does apply to copyrighted materials: If you do not agree to the conditions attached to the sale, then do not agree to the sale, and find an alternative seller whose terms you can agree to. I'm not sure why this seems like such a foreign concept to people - certainly there are tons of examples of how the sale of goods and services work with which you're familiar... why does this one, in particular, seem to be such a foreign notion?
The "it's free to copy, so you haven't lost anything by me copying it," argument is nothing but vapid post hoc justification for taking something that doesn't belong to you. The "if you sold it to me, I can do anything I want with it, forever and ever, amen," is just willfully ignoring the explicit terms and conditions of the sale you engaged in.
Let's look at it from a different angle: what prevents me from grabbing some GPL3'd source code, stripping out all license notifications, adding a bunch of proprietary customizations to the code, slapping my own name in the headers, and selling it as "Americano's Super Duper Awesome Appliance," and making billions while contributing exactly zero code back to the FOSS community?
What's that you're shouting? "GPLv3 has an Anti-Tivoization rule, and you can't do that!"?
Well, who the fuck is the FSF to tell me what I can and can't do with my legally acquired copy of that source code?!
(See now why you need copyright, and anything you do to undermine copyright is only going to hurt you in the long run? Just because something's easily copied doesn't mean you have an absolute right to do whatever you want with a copy of it.)