Here's the official line: "The Rolling Stones have never given permission to the Trump campaign to use their songs and have requested that they cease all use immediately."
So the answer is simply, they aren't.
"The Rolling Stones do not endorse Donald Trump,” the band tweeted. “ 'You Can't Always Get What You Want' was used without the band's permission."
I guess Trump does whatever he wants with songs and to pussies.
Short Answer: Maybe it's useless to get into a legal battle with him. He does have the most luxurious, most fantastic, most expensive lawyers, and those lawyers have the best words.