The reason is that the tuner comes into the box as FM multiplex, and needs to go through the DSP (and hence to the speakers) in order to split it into left/right.

In *theory* you could hook the multiplex signal up to aux in externally and get a mono sample of the current radio without affecting the currently playing audio, but it may be the wrong level and adversely affect biasing of the real FM/AM input. I've not tried it.

Hugo