It's simple: PayPal is the easiest way for us to pay authors from around the world. PayPal works for most countries.
So using PayPal for all purchases, both PayPal and credit card, is the simplest thing for us to do.
We're a Canadian startup which charges in USD, and using PayPal lets us pay authors without converting between USD and CAD, etc -- we can just collect USD on purchases and send it to our authors without any middlemen other than PayPal.
PayPal is the only way to receive royalty payments from Leanpub. We pay hundreds of authors every month, and we need this to be as automated as possible.
If you find PayPal distasteful, one thing to keep in mind is that if it wasn't for PayPal, Elon Musk might not be rich and we might not have SpaceX and Tesla.