EVGA isn't allowed to either if you're looking at it from that point of view.
They aren't selling this on Titan X, they're selling it as a stand alone aftermarket cooling solution.
And if Asus, Gigabyte, MSI, Galax, etc wanted to sell their heatsink as a stand alone cooling solution, or bundle it with the Titan X, Nvidia probably wouldn't stop them.
It's the same thing Gigabyte did with the Titan black, where they included their Windforce heatsink in the box but you had to put it on yourself since Nvidia won't let them sell a Titan with a custom cooling solution already on it.