No doubt that's what many of the companies do. But I don't know if I'd call it a scam when they ask for personal data and you give it to them, and then they sell it. Somewhere will be a privacy policy when they ask for your details (by law) and buried in that mess of legal words it will be spelled out.
Personally I have tried some of them and have had limited success on getting gems - but history has taught me (seeing these kinds of offers before) that I never chose the ones that require I 'sign up' to anything. If it's not just something simple like going to a page and reading an article or something, I don't go for it. Seems to have saved me a lot of grief.