I suspect the logic is, 25MPH (40km/h) is residential speed limit. In a school zone, the speeds are lower because they want drivers paying extra attention. Setting CC for 15MPH or 30km/h removes a degree of attention in an area where extra attention is warranted.
I don't mind the 25 MPH threshold for CC, but it would be nice if CC is set for >25MPH to be able to engage (and accelerate) before reaching 25MPH. In areas where 30-35MPH speeds with lots of stop signs or lights, the manual acceleration to 25MPH, then engaging CC gets annoying.