Just for clarification, there are two separate questions at issue:

1) If AS should have charged me excess fees in this case. This isn't my main question, but I would say no, because elite status with a particular airline overrides standard policies in my experience. An airline may only easily be able to determine my status with them; how would they be able to verify if (e.g.) I had elite status with every downline airline as well to waive all fees down the line? This is why we have unified IATA/DOT rules which both specify (with different calculations) that the bag-tagging carrier is the one to charge the fees.

2) If SU had the right to try to collect fees from interlined bags already en-route. This is a strong no for me -- that's the entire reason why interline baggage agreements exist! This could even lead to a carrier double-dipping, if they were to both charge AS under their baggage agreement and also charge me directly. It seems obvious to me that the carrier which issues the bag tag is responsible for collecting any fees from the passenger, and for dealing with the downline carriers directly under their contracts. There are well-known examples of airlines that elect not to participate in any interlining at all.

But I'm happy to hear someone convince me otherwise. After all, this is FT!