They are waiting for the BTC to disappear. They can't allow something they can't control. And BTC being decentralized is their main fear. They have no control over it and they can't track your transaction if you choose not to disclose your information. This is the sole reason why the government is not allowing BTC to be legal in the first place. And to cover up that, they are showing these lame reasons.
I came across a discussion on my local board today and @Little Mouse has a great example on this.
The translation is somewhat like this:
Pablo Escober owned 30 billion dollars at that time. It is told that, when his daughter Manuela suffered from cold, he burned $2 million in order to keep her warm.  source
All that Pablo Escober had was black money. He was the biggest cocain dealer in the history. He had done all his deals with Dollars or golds. But neither the dollar nor the gold was banned.

In 2009 over $300 million were seized from drug dealers, and obviously they were not Bitcoins.

So the thing is, we need to control criminal activity. Wheater bitcoin exist or other currency, these criminal activity will still continue. Or the illigal transactions. So the Bitcoin is not the problem. The problem is, criminals using it for illegal activites.

So from my understanding from this is, the problem is control. If they can't control something, it's illegal.