The irony is that free software requires copyright enforcement as precondition. I mean, if a company takes a GPL'ed code, modifies it, and sells it to another willing party without following GPL's terms, what does the original author materially lose?
It is not the irony. The core idea of GPL is to fight proprietary software with its own weapon.
Also, I think that you misunderstand the GPL terms. The aggrieved party in this hypothetical case is not the original author, it is the buyer of this modified software, whose rights to four essential freedoms [1] would be violated. These violations have a very real material cost.
BTw, it is common for people to believe that GPL requires publishing source codes on the Internet. It is not so. The requirement is to provide the user of a program source codes, so he can exercise the guaranteed freedoms. You can do it in any suitable form, publishing code is just the most convenient way to do it, but not mandatory.
Hence the clarification "willing party". A company "pirates" a GPL'd code, creates its own derivative version, slaps a sticker saying "This is proprietary code, if you want to see the source then go buy from somewhere else." Someone is willing to buy the product, they are both happy. Just like two internet users sharing copyrighted files.
People can be coerced (fooled, cheated, etc) into believing that they are 'willingly' abandon their rights. This hypothetical example isn't similar to the case we're discussing here in this thread.
Your definition of "theft" is a bit too broad, don't you think?
Usually theft means that the original owner is deprived of goods - they can't use them, can't sell them, they simply don't the the original item. This is simply not the case for digital goods. HBO won't lose money when I download GOT, won't they?
It is totally legal to pirate content where I live now, and where I've lived before. Still, I pay for video games on Steam or for Spotify. Hell I even buy music on Bandcamp. I do this because it's convenient and because I want to support creators.
However, when a creator goes out of their way to alienate me as a customer - by either dropping support for my platform, or by region-blocking - I happily put on my pirate hat and get what I want for free. There is no other viable choice for me.
Steam is successful not because piracy is illegal. It is successful because it made buying games more convenient than pirating it. It is far easier to click, pay, get content than torrenting stuff and finding correct cracks/keygens, etc.
I have bought a great deal of games I played in 1990s (pirated, of course - there simply did not exist a way to buy games legally in Russia at that time), just because I liked them back then.
Also, please stop calling copyright infringement 'piracy'. Piracy is a dangerous penal crime involving violence and an open theft of possessions. It has nothing to do with copying information.