Not trying to defend the decision, but as far as I know, the reasoning is that if you charge significantly less than you could, it might be because you have other undisclosed agreements with the tenants, like them doing some extra chores for you, repairing the flat or something else. This way, you could avoid a lot of taxes. The sentence also doesn't seem to be a fine in the narrow sense, but rather a demand of additional taxes. If I'm not mistaken, it's perfectly legal to charge very little for a flat, but you still have to pay taxes as if you would have rented it out for a regular price.