During cleaning, alcohol vapours are produced that waft towards you when you open the cabinet. The manufacturer probably noticed this and banned it immediately. I could imagine that the alcohol attacks other things in the cupboard, e.g. medicines.

the cleaning station has quite a large circumference. Depending on the depth of the mirror cabinet, the station will not fit in

I have been using the cleaning station in a mirror box for over five years (previous and current model). Haven't had any problems so far and no discolouration or other stains have appeared. So it does seem to be simply a precautionary measure on the part of the manufacturer...