There are a couple of reasons to core out a part. With high volume production, even a few percent material savings can be worth big bucks. Another is that plastics shrink when they cool, thick sections shrink more than thin.

Getting the light to refract thru the part, more evenly illuminating the cap, is a really complex optical problem. I think that's why most backlit buttons are translucent instead of transparent. Even then odd geometries can lead to uneven results.

Unless the casting shop thinks filling in the button would cause the outward appearance to suffer, the cost of filling them in will likely be lost compared to the scrap. I seem to remember a picture in one of the galleries showing a bin full of faceplates that hadn't fully filled the molds.

A tip for any one with hollow transparent buttons now. If you want to fill them in, you will need to fill them in with a material that has a matching Index of Rrefraction. Otherwise the light will still bounce around inside the button just the same as it already does.