Thing is, they actually changed their TOS around a year ago to make this "possible". Before that, the rule was that only the TOS you agreed to when downloading a Unity version is applied regardless of changes in the future. So even if you had read their TOS back then Unity still would have tried to pull this off even though it's most probably not legal.
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It's legal to change your TOS at any time but previous users have to both be informed of it and agree to it. It's basically like renegotiating the terms of a contract. Even if you change your TOS, the company still has to honor previously made contracts otherwise they'll have a legal issue. Where they get you is they will send you the revised TOS either through your account or when you use the engine, and if you hit that agree button well now your contract has been legally changed. Which is why I advice people to read any and all agreements sent to you regardless of how long. If you do not agree to the change, you have a legal leg to stand on when stuff like this happen. And if they deny you service because of it, you have a viable lawsuit on your hands.