An ip capable device doesn't have to implement ping at all.
Especially very simple devices, with very limited functionality, don't do anything not strictly necessary for what they need to do.
It just keeps the software simple (less potential for bugs) and as tiny as possible (less ROM/RAM needed). As a result it will be cheaper to manufacture (in man-hours for programming/debugging and in material costs).
In bigger, more complex equipment the ip-stack is often bought as a, ready to go, commodity-module and the RAM/ROM needed is usually already present because of other requirements.
Then there is often a full ip-stack present, with a ssh or web-interface for configuration and other niceties, because it doesn't cost any extra to implement.