Illegal is the wrong word, unethical is a better fit. As per IATA regulations, if AA is not on the itin, and I am not acting as the GHA, and I haven't received permission from the ticketing carrier to use their plate, then using an interline agreement to lower the price of my clients tickets is just plain wrong. I don't need to expose myself to debit memos or BA turning off my ability to sell them at all. Many of my corporate clients are BA SILV/GOLD and loss of BA inventory would hurt.
Let me also offer another scenario. Since IATA mandated everything to be ETKT, the airlines no longer take change fees on MCO's, but rather reissue the etkt with the appropriate additional collection. Lets say I ticket my passengers travelling on BA on another carriers stock, AA for example. If my client chooses to make any voluntary changes to that booking they will have to do so with the issuing carrier. Not a big deal when major cities are involved where both airlines have airport representation. But good luck finding the AA desk where BA is their sole representation. As soon as that ticket is reissued onto BA stock, BA has the right not only to collect the change fee, but also the undercollected fuel surcharge.
I would rather start of the whole journey on the right foot. If the AA ticket desks at the airport are no longer doing it, I'd say the jig is up and they are closing the loophole quickly.