It's nothing to do with Thai customs, it's Amazon that charges the fees. When I pointed out to them they were adding import duty fees to items that were under the duty threshold here, they said, "Don't, worry, you will get your money back within 180 days", as if that made everything okay.
And yes, if you don't check what the applicable rate of duty is before you order goods to be sent here then yes, it is your fault for being lazy. Nothing to do with whether the duty is 'fair' or 'justified', it's to do with accepting responsibility for what you're doing, like any adult with a functioning brain.