dstiles - 9:43 pm on Dec 9, 2012 (gmt 0)
gzip isn't the only compression method used by web browsers and other agents, although it is the most common.
Non-encoded content is sometimes requested by legitimate non-browsers or browser accessories such as anti-virus tools, which often cannot manage compressed files (it takes time to de-compreess them, which slows down the final browser view).
It is quite possible that desirable but small-time bots may not handle compression.
And as wilderness points out, it isn't a single amazon bot but dozens (or possibly hundreds!) of non-amazon bots using amazon cloud space.