I can buy this if somehow someone illegally obtained your copyright.
Other than that, you don't lose your right to exploit your work. Someone else is exploiting your work in addition to you. They are infringing on your copyright, not stealing it.
Depends on the type and scale of infringement and gets slightly pedantic: if I can no longer workably exercise my right -- if the infringement is too widespread, say -- do I still have it?
You're conflating the right of control of exploitation with copyright, which is a legal creation to protect that right.
The two are close, but not synonymous. Take photographs: the exclusive first-use of a photograph is a big thing. If you publish my picture without my permission, my right of control is gone.
I can use copyright laws and the courts to try and get economic redress for that, but nonetheless I no longer have the ability to decide where and when that picture is first published; the ability was stolen from me.
Yes, I assumed you were speaking of copyright. Copyright gives you a monopoly on your work. Is this what you mean by the "right of control of exploitation?" Is there another law that gives that right or are you speaking from a moral standpoint?
Copyright doesn't prevent copying, it just gives you a way of redress.
UN Declaration of Human Rights (ratification varies by country; your mileage may vary) enshrines a right to control of the exploitation of your works, yes.
The exclusive right to control the exploitation of your creations is something that can be stolen though, technically speaking. Because if someone else appropriates control over what can be done with your work, the exclusive control over it is taken away from you. By the definition earlier, that's theft.
The definition given earlier was: "A person is guilty of theft, if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it"
If I take your pen, you no longer have it; thats theft.