That is because the penalty has to be a light one. You have to give people a carrot in order for them to make a voluntary disclosure. There really is no other way to go about it. Otherwise, you have to put an end to the voluntary disclosure program.
But it is still a good question. Of course, in situations like this, it is difficult to tell people that they are going to get absolution. From a political point of view, or even from an ethical one, there is some difficulty with that way of thinking. But the whole voluntary disclosures program essentially rests on it.