This has been true for just about every US-based airline since revenue-based models were introduced. UA made some changes to mitigate it but it's still true there, just to a lesser degree.

You earn more miles / status qualifying miles / LP / whatever on cheap tickets by flying with partners. This has nothing to do with AA, it's true on DL and also true on UA (though less so since they capped partner earnings) and it has nothing to do with the LP system. It's been true for years and will not change while they continue to give distance-based credit for partners.

FWIW you can earn 30+ miles per dollar spent on good premium economy fares.