I understand where you're coming from, but can't fully agree with you.
First, Stack Overflow contributions are licensed under Creative Commons. So monetizing them is explicitly allowed.
Second, information is not "stolen" nor "goods". Copyright law is completely separate from physical property laws, so even if you could make a case about fair use of training data, copyright-ability of model weights and AI generated content (which I agree are still legal gray areas) and therefore whether or not the "Share-Alike" CC clause is enforceable in this context, it would be an entirely different argument from whether the whole industry is somehow entirely morally bankrupt.
Third, given that this is unpaid work made voluntarily by users of the platforms (Reddit, SO), why is it any more acceptable for these platforms to lock it up and monetize it than for AI companies?
I think it's completely reasonable to charge for API access, particularly above a certain volume, but not because these companies have a right to protect some sort of "intellectual capital investment", but rather because the server costs of processing the requests are not negligible.
If anything, this situation really separates the wheat from the chaff in terms of what pools of open web content are truly "open". If the platforms hosting them expect to retain control of their "investment" can they really be said to be open?
I understand the irony, given that OpenAI's own name is somewhat at odds with its practices (of merely providing open access versus truly releasing everything as open source) but I think the reasonable solution to that conundrum is something like Wikimedia Foundation, Internet Archive or maybe CERN for AI, not giving up on free, open content just because it might feed a giant private brain.