Well, I'm not as torn as the author. They are altering the original content of the author, even if the result is the same. Imagine that someone's religion believes that you go to hell if you use affiliate links... posterous would have condemned them! (dramatizing works)
That's a bit silly, their whole draw is in the fact that they change your content. If you have a youtube link, they turn that into a youtube embed. If you have http://... , they change that into an anchor. There are dozens of examples of content that you can put in an email and they will transform into something more useful, and this is generally regarded as being quite valuable. If you have a problem with posterous…
Having a paid version which would disable this would be another nice option for them to provide.