Brandon Savage has a reminder to all of the developers out there about working with input from your users - the burden is on you to format the data correctly, not them.
The simple answer is that whomever designed the form decided to place the validation of the data, and its massaging into the proper format, onto the end user. But there's a more complicated issue at hand here: the fact that the developer either felt it wasn't his responsibility to do the data formatting, or didn't realize that not everyone would think to [input the data] way he does.
He suggests that it's more out of laziness on the part of the developer to want the user to give them the data in the format they want when, really, the application should take reasonably correct (it has to match the type they're looking for, after all) and massage it to what they need. If it doesn't cooperate after this, only then should they toss an error. His examples touch on phone number and date formatting.