I guess we've all been kind of spoiled by GOG's fantastic tradition of adding so many goodies to their game releases, including soundtracks.

In this we agree.

Well, they add the goodies for the games that had that extra content included on enhanced versions of the releases. As it's the case for older games, they don't always deal with copyright issues with those soundtracks, so they can add them. But for newer releases, unless they sell an edition that include soundtracks, they are not gonna add it as extra.
Take as a quick example Hotline Miami 2. They have the Special Edition that include the soundtrack, but the basic edition does not.
Fallout. When i bought it, few years back, i received goodies, manual, bible, wallpapers. It was probably at the same price. People that are getting it today, are getting the standard edition, without soundtracks and all that stuff, probably due to some contract issues with those extras.

Steam will continue selling soundtracks separately and i think it's a good thing. They even have an player included in the client. It's handy, especially for people with Steam Machines, they can download and listen to the soundtracks on those.

Even though I think it's stupid, I can sort of understand why soundtracks are sold separately, I just find it baffling that they're classified as DLC, making it necessary to own the base game in order to purchase them.

Oh... I just realized that. That, indeed, it's stupid.

I'd rather be charged less for a game sans soundtrack, than charged more for the game with a soundtrack that I'm never going to listen to.

Put another way, I'm glad soundtracks are often sold separately, that way it's optional.

I think you will find same thing happens on that other store.

Hah! You are most probably right.