I had the same thought, and I tested it; I believe (but am willing to believe that I am wrong, as I didn't watch the network traffic itself) that it is actually hijacking the http(s):// URL.

What I did: I went to one of my websites from the Kindle Fire, and sent the same Accept and User-Agent headers (the only ones that seemed to be special) to https://market.android.com/; there was no redirect returned: it simply gave me the home page. However, going to that same URL in the Fire browser application immediately loads the Amazon App Store.