Grey market tractors were originally sold in another country and then imported without going through the manufacturer's distribution network. There are numerous other examples of similar occurrences. Cameras (and electronics in general), jewelry, books, and an array of other goods are all imported and sold in the US after being sold elsewhere first.
There have been several lawsuits and injunctions over time. Kubota managed to get an injunction prohibiting all importation of its tractors. Yanmar USA has filed a multitude of lawsuits against those who sell tractors and even only parts for them. The basic premise of these lawsuits is that the tractor manufacturers (Yanmar and Kubota in particular) hold that it is illegal for a person to use the word Yanmar or Kubota in an advertisement selling their tractor. Further, they say it is illegal for a person to sell parts that may fit a Yanmar/Kubota tractor or advertise for any support or sales of them in any way.
It is unclear what the status of these injunctions and lawsuits will be after the US Supreme Court's decision in Kirtsaeng v. John Wiley & Sons. The decision left some amount of wiggle room open for both manufacturers/publishers and importers to reinterpret what they do and how to implement their future plans.