Tom, it is not as cut and dry as that. The legal issues with intellectual property often become messy with HOW the brand is represented, or HOW the copyrited materials are used, not the fact that they are used. There are some legal ways to use things and there are some ways that run afoul of the law.
As none of us are privy to the actual documents or findings, we can only presume that if Deere put legal pressure on Mahindra, and if Mahindra pulled the advertisements, then Mahindra probably crossed the legal line in HOW they used the representations of JD.
All fo the tractor companies have "comparisions" to other brands on their websites, that is within legal bounds. Also realize that Mahindra has prior ads which showed its products with Deere and Kubota and those ads ran for a long time, so it was not simply a matter of using other brands that set off the problem. Truck companies compare their vehicles all the time, as do car companies. But again, it is not illegal to make comparisions, but it is illegal to make false claims about other companies, misuse likenesses, abuse trademarks, etc.