This isn't unique to *A. OW does the same thing, with BA being the most notorious for not accepting anything in lieu of a real card. Airline computers often print the status a ticket-holder had when the ticket was booked, which may no longer apply. Also, it's easy to alter a boarding pass to show status: check in at home, get a PDF instead of printing directly, and alter it in a graphics program - or any of several other ways. (If you're concerned with using a falsified document to board a plane, print two: one legit, one that says you're super-diamond-platinum with three oak leaves.) Combined with these is the fact that the staff of many lounges can't check partner airline status via their computers. Bottom line: things will probably stay this way for a while, so carry the card or be prepared not to be let in.