this topic has come up before.
a lot of people don't seem to understand:
technically speaking THERE IS NO VOLUME inside digital signals such as HDMI and toslink/optical/spdif. which is why some devices like ftv or ouya "don't let you change volume". which is where the amplification app comes in.
some devices like google or apple, do offer "volume" - but they are not actually changing the volume. they are doing the same thing as e.g. this amplification app: modifying the waveform before it gets embedded in the bitstream. depending on the software doing it, this alteration cpuld be not very noticeable, or it could be. depends on how much you distort the waveform. most people probably won't notice, or care.
with digital transports like hdmi and optical, the idea is simplicity, cleanness, and purity of sound.
the idea is that you connect to a receiver that uses its DACs to output to speakers. years ago, you had to pay a lot more for receivers with better quality DACs than the cheaper ones. these days, most of them have had decent guts, not just the expensive ones.
a lot of people are just going to connect their set top box directly to their TV. which, I don't know offhand what sort of guts they have, but it's ?probably? not the best precision. not to mention the TV built in speakers probably aren't exactly high-resolution either.
so it's probably not so big a deal, that some people want to have "volume" on their set top box, rather than having to use the TV remote to change the TV volume.
not sure why amazon or ouya would not include amplification. maybe it's easier to deal with customers complaining "bbbbbut... there's no volume setting" than it is to deal with complaints about crappy sound in the case the the amp software broke or somehow got messed up or they turned up the volume too high or certain media confused it too much,......
all that said, my KFTV is hooked directly to my TV. which has crap speakers and probably a crappy DAC. lolol.