HI,
It seems that the Agreement Apple and AIM have about Apple Issued IDs being valid AIM Screen Names as well is now so old some staff at AOL/AIM are actually unaware of it.
In the past I have actually come across Apple Documents and Knowledge Bases articles that contradict each other with regard to whether AIM actually "know" the Apple IDs.
The closest I can come to a work model is that AIM do have all the names but don't have the passwords.
This means the AIM Server then have to contact Apple to get "Shown" the password.
This is somewhat "proven" by iChat 6's and Messages dual Login to AIM and Me.com that allow the password to be seen meaning that iChat 5 cannot use @me.com names from iCloud.
Apple are not the only people to have such an Agreement with AOL/AIM.
Some United States Based ISPs (that issue emails) also have agreements.
The only route I know is for you to go back to AOL and explain very clearly and precisely to the person that they are a (insert words of your choice here) without actually saying out loud they are stupid and point out that yes it is an Apple Name and yes there is an Agreement. This will mean there must be somewhere the account got flagged as Suspended - otherwise you would not have got the Suspended Message and can they Undo that Suspension as only someone at AOL/AIM can.
I would also use this page to "report" the fact to Apple
http://www.apple.com/feedback/ichat.html
Unfortunately this feedback page has always suffered with being behind the times with OS versions an iChat versions.
You will have to spend some time entering the info in the Free text box.
8:27 PM Thursday; September 6, 2012
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