Probably because there is enough time in the schedule to add an additional leg onto a transatlantic segment.
US Airways is also probably training crews and needs
the largest aircraft it has on routes that are popular
during the winter. PHL-MIA and PHL-SJU seems like
a market where demand for seats is strong. Besides,
this is US Airways. They have a lousy product, bad
management, and I would not put it past this airline
to do something stupid.