Yeah, Posterous are playing a little bit rough, but Twitpic shouldn't respond to that by making it look like they want to make it difficult for users to export their data. If something Posterous have said is untrue (maybe the dying platform part) then call them on it - post growth stats, demand a retraction, whatever. Don't alienate your users to get back at another company.
"Of course Twitpic should allow export, but Posterous calling others "dying platforms" is an unnecessary dick move. Hence, pettiness."
Here's where they used the term "dying platforms": http://blog.posterous.com/make-the-switch-to-posterous