The key here is that most domestic AA flights that are bought as BA codeshares as an extension of a transatlantic Club World flight book into a business class bucket despite the fact that AA market the flight as having no business class cabin. So if one buys LHR-JFK-LAS from BA, and the flights are BA marketed, the JFK-LAS will be in AA's first class cabin but tier points will accrue as if it were a business class flight (140 TPs). Buy the same ticket from AA, with AA marketed flights, and that same segment will book into a first class bucket and earn commensurate first class tier points (210 TPs). As such, for those running for tier points, or interested in maximising them, it can make sense to buy somewhere other than ba.com as there's no way to get ba.com to sell AA-marketed flights on such itineraries (that I know of).

To speak to the post upthread about how AA markets their cabins, they are actually more consistent than some of their US-based competitors (DL is one of the worst offenders there). Here is how it works 100% of the time:

• Flights sold as three classes are obvious: first, business, economy. Typically the only domestic routes sold with three classes are JFK<>LAX, JFK<>SFO and MIA<>LAX, though there are exceptions.
• Flights sold as two classes that take off and land in the 50 states are always marketed as first and economy (this does not include territories like Puerto Rico--just the 50 states).
• Flights sold as two classes that are not wholly within the 50 states are always marketed as business and economy.

These rules are axiomatic and are applicable irrespective of the type of aircraft flown.