FireStick ( and / or ) FlexBox volume control ....

One way to find out. I know my friend's Kodi box would try to turn the volume down on Dolby Digital streams, which made a bad sound. If the device is only outputting PCM stereo, that should be OK, although you'll sacrifice some bits of resolution.
 
Because having volume control on the device and also on the TV would absolutely confuse the average consumer. It's an added variable that 99.99% of people don't need.

Imagine having two ball valves in series on a water pipe. You open the first valve expecting to get full flow, but either nothing comes out of the pipe or the flow is lower than expected because the second valve is shut or just slightly open. I've seen that scenario confuse more than few people.