It's one of those ideas of legislation that appeals to the voter. The public thinks 27% of the people on benefit commit fraud. So, lets get the bastards.That it is less than 10% is just not sexy to tell to the public. It does not bring in votes. Tax evasion is a sport, no one would think of tackling that harder, because that would cost votes.
So, it's not about common sense, it is about appeasing the voter. Stupid idea.