The Id and Id Pi has its own license built into the device (or in this case the SD card image used for the Id Pi) and it can't use a normal MC Windows, Mac, Linux or Master license.
JRiver doesn't offer image downloads as they have to customize each Id/Id Pi with its own unique license, so they only offer fully baked Id Pis (includes a full Raspberry Pi 3 Model B+ build with the microSD card that has the OS+MC (and license) included or just a microSD card with the OS+MC (and license) pre-installed that you can pop into your own Raspberry Pi 3 Model B+ for example.