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.
Why not metaweblog api? Posterous has api for only posting data. I have not seen one to fetch. If you are the best kid in town why do you need to be afraid of others?
If Posterous is open about data portability, customers will be happy as well.
I am simply asking Posterous to take moral high ground. Give a feature where people can export their posts in standard data formats. then go ahead and target your competitors.
You can easily get all the data from XML feeds. Or you can use their API to get all the data out through XML or JSON. Both are standard formats. I am pretty sure no one offers a export my data function. Not even Tumblr.
Posterous supports XML/JSON but that's not good enough for you since they don't support metaweblog. While we are at it, I would want Posterous to support .txt, .ini, .html, NewsML and any other plethora of the formats that exists.
Your argument is flawed and you are changing your base premise when called out.
