I’ve honestly not heard of profs who directly offer fee-based research experiences to the public (e.g., high schoolers). It would be viewed very poorly in my professional circles. I would be appalled if any of my colleagues did that. I don’t consider it ethical.
Of course, my brain’s output is mine (IP thorniness aside). So I could be a consultant in my free time, offering my time/expertise to outside parties in exchange for money. That’s typically quite expensive and customers include companies, lawyers, etc.
But a high schooler doesn’t need my expert opinion on anything. They’d hire a less expensive person for anything they’d actually need, such as tutoring or advising. If they want to do research in my lab, they’d be there via the university or another official program. They can work as a volunteer, or I can pay THEM. They can’t pay ME directly to work in my lab. Since I’m a biologist, (nearly) all my projects require physical resources.
Depending on the discipline/school, a “pay to play” scheme for high school “research” may run afoul of faculty guidelines or even laws. My lab resources are purchased by my institution and federal funding bodies. I didn’t purchase any of it with my personal money, except my laptop which I specifically want to be my own. If I were receiving money from an outside party to use any of my academic lab resources, that would all need to go through the university – I couldn’t get paid directly.
I happen to know a professor who is currently in deep doo-doo with their institution and federal agencies. It involves billing things related to federal grants to their company, which then goes to them personally. They are about to get busted, and their career is over.
You have to be super careful with this stuff. And it’s just wrong, besides. I also saw that tweet about the emails. I have yet to get one of those (though I get the publishing ones every day). If it isn’t a phishing scam, and money is truly going to profs, it’s gross and I hate it, and so would my colleagues. I can imagine that some people who are unscrupulous or really hurting for money would do it, and it’s too bad. If you want to make a self-employed career out of advising high schoolers, go for it. Just don’t do pay to play as an academic researcher.
Most people don’t understand the reality of high schoolers doing academic research. I think this explains it quite well, and is relevant to high school research (scroll down to the sections called “science fairs” and “starting research”).