Because what are you going to import that file into? Everything needs a custom import format. Making the API available makes it easy for someone to write something that exports to the format needed. Do you think Posterous should host a different file that allows easy importing for all of its competitors? If the competitors care, they can make a tool that imports into the format they desire. Likewise, if a user just wants a text file of their contributions, they are free to obtain one. Someone could write these tools and publish them, but it's a little over the top to expect Posterous to write an exporter for everyone else's custom formats.