> Why are proprietary software developers entirely unbothered by this?

Because the chance of someone auditing the actual source code is near zero, but for open source software it's near 100% that someone or, worse something aka an AI bot looking for a revenue stream, will have a look at your code. And EVEN IF you get caught stealing by whatever random incident (say, your code repository being 0wned) - it's a matter of a settlement payment. If you are not a company, but an individual person... well, just look at what happened to Aaron Swartz RIP.

And for the WINE/ReactOS/console homebrew folks, it's vital to be able to claim no code contributor ever got exposed, even indirectly, to proprietary code.

I heard many cases of LLMs outputting their training data verbatim; they sure don't check that such data can be legally used by you before doing it.

As others said, for closed source software it's a lot harder to detect plagiarism.

And it's in general very hard to detect plagiarized code; there's some product that does it, but it of course has only a partial inventory of the existing open-source code.

I think that some big company does use those products, though.

They did in 2023 and the AI thieves built logic to rephrase the plagiarism so it isn't immediately detectable.

Proprietary developers don't care because their code wasn't trained on. Is that a trick question?

They do care now that reverse engineering is possible.