At the end of the day that's a tricky one really, because it all boils down to overall price, and for many the price is the important bit.

There are two ways to look at the inclusion or exclusion of extras like soundtracks and artwork.

Many games at GOG have a deluxe version which comes with the extras.
So you could see that as paying a bit more to get the extras.
Or you could see it as paying less to get the basic game without the extras, that many consider unnecessary.

So some would say, that maybe you are getting an extra discount if you don't want the extras.

Anything that is part of the full package of a game contributes to what you are going to be charged for it, and soundtracks especially usually have a different license and their creators require recompense.

GOG ultimately want to sell a game they stock, and if exclusion of extras means the price is cheaper, then they could sell more of it.

P.S. A good number of games actually have free extras in the Free Games section of GOG, where you can see quite a few Soundtracks listed.

GOG controls all things forever and ever, thus we can blame them for all things unto the end of time.
Yooo, that'd be sick.