Although unlike a naval vessel, Starfleet ships have multiple engines for different speeds. You don’t want the pilot to be confused and rack the ship up to Warp cuss when you wanted full-impulse only.
It is also possible “Impulse speed” is a standardised speed in its own right, similar to how someone might say “object approaching at warp-speed, on intercept course”.
Though they have made odd decisions before. IO is generic and widely-used enough that it doesn’t seem implausible that they might change it to not be tied to the Indian Ocean territories.