The problem starts with the fact that the device doesn't have any way of showing that it was modified, and some of the frequencies involved are license-restrict

The problem starts with the fact that the device doesn't have any way of showing that it was modified, and some of the frequencies involved are license-restricted (especially in 5GHz wifi bands - 2.4GHz is dumping ground free-for-all because of aircraft ovens anyway). So, let's say you modify something with your own firmware, break rules about ISM spectrum - or worse, mess with SDR hard enough you break some licensed spectrum, and upon investigation FCC certification marks are found and the number. Since certification points to vendor, vendor now has to explain why their device went outside of those limits, and might or might not be able to prove that you ran it with unlicensed firmware. So an obviously home build device will go under §15.23 easily, but inconspicuously modified commercially sold device won't - without possibly long court case, that is.