Being from Philly, I have had similar experiences. In my case, my website was registered to a Philly address, so they automatically sent out the "you must have a business license" letter. I was making no money. It's their number one automatic letter. My 3 year old daughter got one, once, even. (we're not sure how, in that case). All a person has to do is call the number on the letter and say "this isn't a business" and they'll put a check-mark in the "not a business" box. They don't care about the miniscule $50 every two months businesses. Just the profitable ones.
This is why, when I lived in Philly, I always had a PO box or location outside the limits for business/website purposes. Mostly to avoid the hassle. The current township I live in is MUCH more reasonable, thankfully.