My thought is that it is not an optional feature.
It is the normal way the MCP buttons work. Every FD/AP mode engaged that illuminates the "ON" light on the button (or the button's symbol light in new MCPs) can be deselected.
It simply works like the other buttons, it's nothing optional or odd.
The airline I work for has, in the SOPs, the procedure to deselect the Speed mode on the AT during approach and landing performed by a pilot under training.