It certainly appears as a principled stance now, but they opted out of OTAs back when any ticket on any airline got you two checked bags.

They didn't want to be easily compared back then on a level playing field. So it's a bit surprising that they want to be easily compared now, but the industry is also different.

Most likely if there was a principle, it was that they didn't want to pay Sabre to be in it, especially since Sabre was part of American Airlines until 2000.