My guess is that someone turned on one of his computers which automatically logged him on to the AIM address.
Well, if that could happen while he's away in rehab, I suppose similar occurrences could have happened in the past, while he was sleeping, passed out drunk, etc., and his "dates" could also have access to his AIM account.
Just another glitch in the whole ABC "investigation."
Not making excuses, just being annoying like Columbo used to do.