The fire doesn't have 3G so that's a pretty big hardware limitation when it comes to processing voice requests. Can't really access the cloud infrastructure without it unless you've got wifi.
Probably why Apple only has siri on the iPhone and not pads. Most pads don't have cellular and they're trying to keep a consistent experience. Probably next iPad will be marketed with siri being a reason to spend more on a 3g capable iPad.
I wasn't referring to Fire. Amazon would certainly offer an Android app and their own iPhone app that works on non-4S devices (if Apple would even allow that).
And I have Wifi access pretty much all the time. Who is Apple to tell me I don't need Siri on my iPad while sitting in my own house? I don't get the argument that you need 3G at all.