Right, didn't mean to imply they didn't. But the differences are that Tarsnap is pre-pay[1], while AWS/GCP automatically debit your card.
How many people have several months, or a year, of runway on their Tarsnap account balance? Meanwhile, there have been cases of people being dead for years[2] with their auto-pay agreements keeping everything humming along while they rot. Keep in mind that recurring payment agreements often aren't cancelled when a credit card number or expiration date changes.[3]
Finally, AWS will give you about five months of unpaid bills before they suspend your account and delete your data.[4] I would assume GCP has a similar policy.