Can anyone explain why it behaves differently?
The problem is that it's submitting a POST to the url, not a GET. When you post to that url, you get a 405 Method not allowed. Try this:
CODE
<form method="post" action="http://devsanctum.com/myfavteam/about.html">
<input type="submit" value="Click me!">
</form>
The better solution, of course, is to fetch the specified url using the same method that was used to request the landing page. There's a bug to that effect in our queue right now, but there are a handful of issues around using GET. (Mostly regarding caching and OAuth signatures, some hairy stuff, not impossible, but tricky, and we don't want to break other things by fixing this.)
For now, you can work around this by configuring your server to allow POSTing to html documents.