Agreed, ad. replacement may be derivative-work liability for Amazon; I don't think the case-law is clear.
But they don't have to directly replace ads. in order to monetize ads well. Because they have all of a user's browsing history with Silk, they can target ads. as well, or possibly better than, Google. Perhaps they show ads. at the bottom of the screen, or in the screen saver, or while web-pages are loading, or with location-based pop ups for coupons for a store just down the street, whatever.
In this respect the Fire is a direct attack on Google, and Amazon probably has to take this step in order to be able to subsidize this tablet as much as Google will be able subsidize their tablets. For this reason I wouldn't be surprised if Google soon cuts Amazon off from Android by making more of the OS closed source, and breaking app. compatiblity. If this occurs, expect Amazon to make a bid for Palm.