Why wouldn't they assume that? Amazon surely knows that some people block AWS servers (AWS customers sure know!).
You also have to realize that Amazon gets the advantage of crowd sourcing--in short order they should know who's blocking what and not even have to make the original requests to be blocked.
Not only that, but what's to prevent them from caching content after the first client makes a successful direct connection? After that, they can serve the cached content quickly to other Silk users. Circumnavigating such blocks may even be one of the driving forces behind developing Silk in the first place, since it allows Amazon to access content they can't easily reach otherwise.
I think it will only be available via a buried setting that most users will never touch.
Fire seems to discourage any kind of advanced user, it's the king of "no"
It also brings yet another browser to worry about for website compatibility and will require simulation to test.