As far as I'm aware, Posterous has done nothing to actively discourage migration of information away from their service, and they certainly haven't threatened legal action on anyone for moving their online information around.
Your argument is flawed and you are changing your base premise when called out.
http://posterous.com/api/reading
What exactly are you looking for? We're open to suggestions and will gladly expand our API to keep Posterous open and flexible.
There's no hypocrisy here. Our goal is to let users move into and out of posterous at will, and use whatever service they find to be best.
Thanks!