Isn’t this illegal, in many countries? I guess the rationale, is so that you can tell them they unsubbed, in the past, if they try to sign up again. I suspect the real reason is not so wholesome.

It’s a problem. I once got spammed by the legit admin of a site I had deleted my account from, years previously, because he scraped the DB. He used it to try ginning up followers for his new personal Twitter account.