I have myself been in a landing where the ABS went haywire and the plane came to a grinding jerking halt (including white knuckled FAs). PAX would have no doubt been falling over eachother if standing in the aisles at the time.

I doubt any AA pilot would execute an extra hard braking maneuver so as to literally (!!) injure passengers. But enough to drive home the point that standing is hazardous to themselves and others, why not?