How so? It's as trivial as creating a new email account to create a new Paypal account, and even if they require credit card only Paypal accounts that can be easily fraudulent too. Fraud via paypal is just as rife as fraud without.
For example, an annoying system which will decrease chargebacks is "If your IP address goes to a country other than your billing address, shoot first and ask questions later." It bites me all the time, but there are sensible business reasons for it.