I remember, back in the pre- internet days, there was a similar situation at THE COMICS' JOURNAL, and the guilty party claimed that it was allj unintentional- that he literally didn't realize that he WASN'T the author of the specific phrases he was writing until it was pointed out to him. Once he was made aware of what he'd been subconciously doing, it was like a light being turned on, and he confessed everything and quit the biz immediately.
Not sure I buy that (and it's still not an excuse for the misdeeds done)... but it DOES explain how someone could think they could get away wit it on such a grand scale- because they weren't thinking at ALL.