Logitech Media Server is free, so that's why it is integrated.
iPeng, however, does not appear to be free (if I'm looking at the right thing) which suggests that Spotify either don't care about this restriction, or they're small enough that they haven't noticed.
It would be nice if the JRiver team could try and get in touch with Spotify and see if there is a way for them to integrate it in the program - but again, I wouldn't keep your hopes up.
However, with the introduction of a WDM driver for Media Center, you would be able to use the Spotify player and send the audio through Media Center's DSP engine.
It's not the same thing as seamless integration, but it's something.