Apparently, it is because all of them "cheated" on government emissions certification tests. Including Mercedes-Benz, which just recently "agreed" – that's the word used by government when it successfully forces a target to submit by using threats of worse repercussions if the target refuses to "agree" – to hand over $6 million as punishment for "falsely advertising" the cleanliness of its diesel-powered passenger cars purchased by people in the state of Arizona.

At the federal level, Benz has "agreed" to hand over $2.2 billion, including $875 million in civil penalties assessed under the Clean Air Act and $546 million that went toward "fixing" the "affected" vehicles. 

Mercedes no longer sells diesel-powered passenger cars in this United States, having been given a strong incentive not to.

Nor, of course, VW – which used to sell a lot of them. Toyota – which sells a lot of them outside the U.S. – may shortly be forced to stop selling them, too. All on the basis of the same claims.

But these claims were always just the excuse. Very much as the claim by a cop that he "smells marijuana" after he pulls you over becomes the excuse to search your vehicle. After he uses the excuse that you were "speeding" to pull you over in the first place. It's really because he – or rather, the government he works for – wants your money. 

And something else.

The excuses aren't reasons – in the morally meaningful sense – for no harm has been caused by your driving 43 MPH in violation of a sign that says you are not allowed to drive faster than 35 MPH. Nor your having "smoked marijuana" – even if you actually have smoked it.