The FSF has an ass-backwards approach to firmware, leading to only these distros fulfilling their requirements.
Their preference for firmware is as follows:
- Firmware that's open source (fair enough)
- Firmware that can't be updated (i.e. devices that are flashed once at the factory)
- Firmware that can be updated (CPU microcode, firmware for GPUs, SSDs, etc)
As Linux includes patching of CPU microcode on boot (to fix security vulnerabilities and bugs) the default build of Linux doesn't fulfill those requirements.