Other than the set screw, which may have red Loctite on it, I'm at a loss as to why would the rear not be adjustable.

I mean, the sight is definitely removable since S&W has replacement sights listed in their flyer for Shield accessories. Since it's removable it can obviously be "moved" as well. My guess is that the screw has red Loctite on it just like my E Series 1911 TA did, which could make it problematic for a non-armorer to loosen it.

P.S. Your highlight color doesn't work well with the light background