I do not know the "official" answer, but I'm sure that the City of Philadelphia has found a way to "monetize" every aspect of operations at PHL, including dragging aircraft around the tarmac. Be glad they don't require that the aircraft-dragging be done by PHL employees, or your aircraft would have been dragged from one side of D to the other via I-95 and through less desirable portions of west Philly, where it would have been liberated of various bits and pieces

By the way, lest people get the wrong idea - Philadelphia is a wonderful city - please come and visit it and enjoy some of the sights. It really has a lot to offer. Sadly, it's saddled with a horrible government and a truly dysfunctional airport with a completely inadequate physical plant and municipal employees and leaders who's attitudes hearken back to the heydays of 1970s European socialism.