Bitcoin transactions are irreversible, but so are cash transactions. Wrapping either currency with a transaction model that introduces a delay between commitment and transfer allows reversing during the delay.

Paypal transactions are reversible because there's a delay between me confirming the order with a merchant and the funds appearing in their account.

Paypal bitcoin would be similar; I deposit BTC with Paypal, and a week or two after I approve sending to a merchant, the merchant gets BTC in their Paypal account.