I wonder if it is because that is a low voltage can. Are all there cans for showers requiring GFCI protection.
Certainly different areas could have their own amendments but NEC does not req. unless the mfg does. This is the first one I have seen but I never use LV in a shower.
Im not sure if all their cans for showers require it or not. The story I have heard about why our state require it is because someone claim to have gotten shocked off a 'shower' light. It was found to have not been sealed properly (have to remove the trim to get to the bulb) and the heat and steam took its toll on the egc, water accumalted in the light and corroded the egc, that created high impedence and wouldn't clear a fault. So the trim/skin of that light was energized with 120volts
Now how the 'accumulated' water stream drip down from the light onto the person during taking a shower and make a 'connection' does seem a bit far to me but it is possible and its cheap insurance to just put it on a gfci. It was said that both the manufacturer and EC was listed in the lawsuit. I don't know if its true or not or if the person won, I heard it was settled out court. This was a some years ago.
Even in areas that dont require it, sometimes its just 'cheaper' to buy the little insurance than to pay a 'settlement' and attorney's fee's even though you were in compliance with the local electric codes IMO.