Really the only thing to do, as soon as you see a "pay with paypal" screen is to go away screaming.
Here's the anecdote: I had had a company with a bank account and PayPal account, then dissolved the company and closed the bank account. Months after that, a former supplier of the now nonexistent company who had PayPal authorization deducted funds fraudulently from the PayPal account, claiming they had rendered services that were never rendered to a company that obviously and provably no longer existed.
PayPal sent the account into overdraft. When they couldn't deduct payment from the bank account that no longer existed, they started sending threatening communications to get me to settle the balance. I took things up with their fraud unit to get the transaction cancelled. Their fraud unit dismissed my case without looking into any particulars regarding the services that the vendor didn't render or the company that should have received services that no longer existed. To them the only thing that mattered was that, years ago, I was, in actual fact, stupid enough to click on "Pay with PayPal", the ramifications being that vendors are entirely within their right to use PayPal as an instrument of fraud and legal intimidation against me. It's your own damn fault, sir, for being so stupid and using PayPal.
Knowing that taking the legal route would have been way more costly than the amount of the transaction, and wanting to sleep soundly again against the backdrop of PayPal sending threatening communications, I wired money from my personal account to settle the balance and jumped through a shitload more hoops to make sure the PayPal account was properly closed and couldn't come to haunt me again in the future.
I think that's how they get away with it: Since the transactions they handle tend to be small, no one will take legal action.