It's a planned feature - it's simpler for the engine to use separate images for now.
However it's not entirely straightforward if it saves you any download size. If you have a sprite sheet like that and a single 16x16 image makes it just over 256 color, you end up saving the entire sprite sheet as PNG-32 which can be 2-3 times larger. Otherwise C2 would've spotted it's a 256 color image and automatically recompressed to PNG-8.
It's possible for C2 to work around that of course, but it's pretty complicated and in the mean time it's working fine soo....