> I can also buy an itinerary where I fly AA on BOS-JFK and connect to BA JFK-LHR, because there's at least one AA-marketed flight on the ticket.
Dunno if that’s a requirement that AA has, but I’ve totally bought 100% Air France metal tickets that don’t even touch US soil on Delta because Delta sold the same itinerary for way less.
You bought them on their DL#### codeshare flight number (what [AA or DL]-marketed means in GP's comment).
I fly a lot of "Delta" flights that are entirely KLM or Republic metal, often buying the Republic flights with Delta points earned on the KLM flights. (NB: the Republic airplanes say “Delta” on the side, while the KLM ones say KLM.)
