I'm not sure, and the server response from your sample certainly does not fit with this idea, but maybe it's because curl uses http/1.0?

Or is that wget that uses 1.0... Fire up wireshark ;)

Edit:

So it was the useragent and accept header that did it. Nevermind my stupid idea.

I think it's the same thing that sometimes redirects hotlinked .jpgs to a gallery representation of themselves: for anything that looks like a web browser, it checks whether the referrer is the embedding page, and if not, redirects to the embedding page.