ET3D: Heh. But seriously, how would a company prove in a court of law who the account belongs to? On GOG there's a birth date field as part of registration, so I suppose that could be used, but say I have a Steam account, what would identify me as the account owner?
Maybe by the credit card used with that account (if I recall right, Steam saves cc information, even if encrypted), or if the case is already in the court as you suggested, that the original owner of the account is dead, which is the reason he/she wanted to transfer the account in the first place. Pretty hard to use an account from afterlife, hence the account could be suspended.
I don't think Valve would necessarily sue anyone from using someone else's account, they would just suspend that account if they somehow found out, or strongly suspected such misuse (if someone complains about the suspension, they can re-check his/her identity then). If there are any cases where a Steam account was suspended as it was thought it has changed ownership, it would be certainly interesting to find out how Valve made that conclusion.
Anyway, I guess that is all irrelevant, as I understood the original question was what is legal, not "what can I do without getting caught?". Similar if he asked whether he can legally share GOG games to all his friends, I don't think the proper answer is "Who can stop you?".