During 2011 I worked for a family owned tire chain as an auto mechanic. My boss, though not much of a technician, was quite knowledgeable on tires. He claimed that according to the T.R.E.A.D. (Transportation Recall Enhancement, Accountability and Documentation) Act, instituted after the Firestone/Ford Explorer/rollover fiasco, it is now illegal in the US to plug a highway tire. This guy was usually right on this kind of stuff. Now I work for a used car dealer who sells a lot of high end German cars. They almost always plug tires instead of using the plug/patch device. A few days ago I fixed a flat on a late model Audi & was told to plug the tire since most of our tire repairs are done for free as a courtesy. I was gonna tell my current boss it's illegal but I,m not positive.
Maybe capriracer or some other tire experts could throw some light on this question.
I've googled TREAD Act & found "everything but" tire repair legality.