There are no confusing or complicated scenarios whereby you could potentially get banned. If you are doing something bad, you know that you're doing something bad, and you will get banned for it. It really is just common sense, so I don't think this is really a factor.

As for whether or not this is legal, I don't know. There are mixed messages in regards to this, with people getting banned for sharing accounts, meging accounts, selling accounts. I understand why this is done, because there is no way to check whether someone stole an account or bought it, etc. It's either this or there will just be no support for accounts, but the e-mail you get states:

Warning: only approve the access for trusted people, your are responsible for what is done with your account.

So that doesn't help. I guess the staff need to decide first