Almost all digital content has been freely available for years. At this point, we most often pay for things because we want to support the developers, not to get access to something. There are exceptions, like online multiplayer access, but OSTs and other bonus content certainly don't qualify. I've always thought of buying these extras as just a way to support a game you particularly enjoyed.
I'm not sure who you're referring to when you say "we". I most definitely don't buy games or other digital media to support the developers. I buy them to legally acquire something that I may enjoy. Sure, for some games that I have quite a lot of interest in, I may buy them to support the devs.. but in most cases, I don't. The BTA bonuses are really just an incentive to get me to pay more to get more content, be it exclusive levels, an exclusive build of a game, and if the music is good enough, maybe a soundtrack, but 99% of the time I don't BTA just to support the dev.