UA, like all carriers, writes its rules broadly and then enforces them selectively. I strongly suspect, but nobody can be certain, that UA could care less about people "managing" their immediate relatives' accounts or their executive assistant using their password.
But, this had all the makings of a commercial operation and it got shut down.
OP doesn't believe that it's worth his time to write in to UA with an explanation. I find that very odd. I have no idea what UA's security folks will do, but I can't imagine not sending in a webform acknowledging the error, noting that no other violations took place, e.g., the miles were not sold or bartered, and asking UA to reconsider.
But, that's up to OP.