If I were the other car companies, I'd want to know exactly how VW was pulling that off. They must have looked into it, and surely they're not as easily misled as I apparently was (how would I know if VW was outright lying about the car?).
This suggests to me that the other car companies must have known that VW was doing something wrong. The fact that they didn't rat VW out suggests to me that they were either doing the same thing (maybe not as aggressively as VW) or they hoped to get away with the same thing. The former seems more likely, and if that's true then I expect this to spread beyond VW soon.