There's no reason for what you've experienced: AA should have fixed the programming on its website to honor its Contract of Carriage a long time ago. This states
When you make a telephone reservation with American Airlines or when you make a reservation via AA.com, the fare quoted will be guaranteed for 24 hours or until 11:59 p.m. Central Time the following day, whichever allows you more time. If you elect to make changes to the itinerary within this time-frame, the ticket price may change.
Share your experience with
http://airconsumer.ost.dot.gov/escomplaint/es.cfm