I suspect it's because they don't have a holster for the M2. I have the C2, which is fundamentally the same body/tailcap with a lower-profile 6P-style bezel, and it will not fit the V25 because of the grip ring. The ring is too wide to slot the light into the holster. Seeing that the M2 has the same ring, it may have the same problem.
SF probably intended these to be carried on the belt with the clip.