You still control Spotify from your phone/tablet/computer but the playback is via your B2 using what Spotify terms Spotify Connect
It is the Librespot software on the B2 that enables another device to see the B2 as a ‘Spotify Connect’ playback device.
The current Librespot implementation only supports Spotify premium accounts. In recent months Spotify have released an SDK to allow Spotify free accounts to use Spotify Connect.
It is up to the individual device manufacturers as to whether they have updated their software/firmware to enable this.
John