as an EDUCATED user YOU accept it, i'm not sure most of the posterous users understand and would make the same decision to user posterous if they did.
this is like saying car companies could sell shitty locks on their cars because they mostly wont be tested anyway, and the driver will have an easier time getting into the car. it's VERY unlikely my mothers car will be broken into just statistically speaking, but hey even if it happens its just one person. not a big deal.
im pretty sure if posterous made it clear how easy this is many users would stay away, just like many people would not buy toyotas if they came with shitty locks, no matter how little they expected to be broken into.