Cause the holster is crap?
I was quite hesistant to insert my M3 into the holster, bezel down, when I received the holster. I just gently tried to push the M3 in, and it chipped off some HA on the crenelated bezel right away, because the springy metal lid of the holster had so much tension. After that, I just removed the clip (you can tear it out-though it may break, as it did in my case), and put some foam or rubber in place to create some tension to keep the light secure.
Not sure how long or how well this will hold up, though. Chances are that the light may fall out if theres not enough tension..... .
I was pretty disappointed by the holster, and the fact that the HA chipped off so easily-just like you. HA may be quite tough when it comes to wear, yet it seems to chip off rather quickly, especially on edges. It`s something that Ive noticed on three of my Surefires... .