The fiat payments are usually not reversible either but it's paypal's system that is the weak link. Paypal doesn't actually take the money back out of your bank account they just minus the balance from your account and then you owe them the money so they essentially take the heat until you pay back. If they handled bitcoin payments it would probably work in a similar way. Either that or they will just hold the money as some sort of escrow until the transaction has been successfully completed.
Actually it does. I sold multiple items on eBay and some buyers submitted a claim that the item was not as promised or not as delivered. Immediately after the claim was filed, the transaction that put money into my account was instantly frozen and I was not allowed to access the funds. When eBay took support of the buyer, the same transaction was reversed.
They can freeze your funds in your paypal account but they can't do anything if you had transferred the money into your own bank account. Your Paypal account balance would go into negative but they can't reverse the actual payment, only you can do that if you choose to pay it which you don't have to and can fight it. If the money is still in your paypal balance they'll take it though.