I don't have a source citing this exactly, but just seeing the news here, I can guarantee it's because Philly pretty regularly brings up lawsuits and sting operations with uberx for 'illegal' operations. Obviously Uber Black doesn't fall in this category because those are licensed drivers. You can really only drop off not because it's 'legal', but because that can't be enforced like pickups can - Uberx drivers (before the ban) would park and wait for rides, alongside legal taxis, making traffic even worse there, and yes, much to the dismay of taxi operations who constantly let city hall know of their grief. I've heard of people being asked to sit in the front seat of UberXs to appear as if they we're just a passenger in their friend's car to be safe.

You can get a Lyft though.