They can be purchased either way.
From the Seeed web site...
The KiwiSDR is a custom circuit board (cape) you connect to the BeagleBone Green or BeagleBone Black computer. You simply add an antenna, power supply and network connection. The KiwiSDR is available in two versions: the cape alone and a more complete version including BBG, enclosure and GPS antenna. Both versions include software supplied on a micro-SD card.