A lot of great information so far in this thread.

It's been a few years since I've regularly used American to commute. When I used to use them they had different policies for different OAL jumpseaters. Some airlines have unlimited jumpseat with American which it sounds like the OP's airline has hence the D6U code the U standing for unlimited. Few airlines however have this agreement with American and are limited to the amount of jumpseats up front on that particular airframe, regardless of the number of open seats in the back that are open. Most cases 1 jumpseat hence only 1 OAL pilot allowed in the back.