> Southwest, the biggest US operator of 737 MAX aircraft, bought the "AOA Disagree" option on their aircraft, and recently added an AOA indicator. Many operators didn't get that option; Lion Air didn't have it. If you have that, you get a warning that the flight control system is seeing bogus AOA data. It is somewhat less of a safety issue if you have the backup systems.

This seems like an important factor. I wonder if that's the reasoning behind the FAA not grounding the planes and assuming most US airlines have this system in place.