Because you don't want to ever store bad data. There's not point to that, it will just create annoying situations and potential security risks. And the best pla

Because you don't want to ever store bad data. There's not point to that, it will just create annoying situations and potential security risks. And the best place to catch bad data is when the user is still present so they can be made aware of the issue (in case they care and are able to solve it). Once they're gone, it becomes nearly impossible and/or very expensive to check what they meant.