First off, management pilots, sometimes known as chief pilots, mainly do paperwork. there are two kinds of pilots, some are good at flying and rotten at paperwork, others are great at paperwork and rotten are flying...rarely do you have both...though some are rotten at paperwork and flying!
now:
why taxi slow?
flight attendants might fall down if you taxi too fast and then suddenly have to stop! lots of paperwork involved, ergo a good pilot doesn't taxi fast.
airplanes are quite good at turning in the sky...on the ground they are just fair.
long taxis at high speeds with frequent braking can increase the heat of the tires and the brakes...on takeoff the brakes might not be cool enough for a maximum effort aborted (rejected) takeoff. cool brakes are good, hot brakes are bad. using alot of brakes on landing and taxi in, can still effect the next takeoff if it is soon after landing. good pilots know this.
and the big one...IF YOU TAXI FAST and can't stop in time to avoid a collision, you might be dead. especially while crossing a runway...though sometimes ATC makes you "expedite" across the runway to make traffic flow.
And sometimes pilots are pissed off about having a pay cut of 50percent for 7 years and want to make mangement suffer.
BUT, American, once a great airline, has mangement / labor problems. shame on management!