Both physical goods, cash and bitcoins once transferred are hard to get back. But this does not invalidate property rights and prevention of fraud. If two parties cannot resolve a conflict by themselves, they turn to a third party. This has nothing to do with bitcoins, paypal or any money system.

Bitcoin is valuable because it gives you a choice: you can either have third party to resolve issues and insure risks, or you can go by yourself. In case of paypal/visa/wire transfer you do not have such choice.