This is "blame the victim". Posting something on the internet does not make it public domain.
"""If I was to place some proprietary code on a public git, or just display it on my public webpage, should I get angry if people take it and don't pay me for it?"""
Yeah, you should.
It's the same if you posted some GPL code on a public git, and someone took it and build a closed system with it.
In both cases, the creator/owner or something did not have his wishes and copyright respected.
The same way we insist on people/companies respecting GPL copyright licenses, we should respect other people's licenses for THEIR stuff.