This is another one of those "is your 'conspiracy theory' filter miscalibrated?" questions. It doesn't take much research at all to find many concrete, documented instances of this, organizations that do it, organizations that you can find that you can pay to do it, people posting their accounts of having worked at one of these companies, pictures of their setups, all kinds of things. If your filter is going "no, of course nobody does that, that's just a conspiracy theory", you need to recalibrate it because it is way off. Yes, people do it, at scale, and there's little reason to believe the stuff you can uncover in 5 minutes of searching is all of it either when there's every motivation for a lot of it to stay hidden. It's not a theory, it's an entire industry.