Its not directly referenced one way or another in the License Agreement, which means its allowed.
Anything not specifically prohibited is by definition permitted.
There's nothing "illegal" about intalling an unsupported OS on a Mac you own. There is no actual law that states you cannot install an unsupported OS on a computer. Hence cannot be illegal.
Its no different than installing Linux, or some other OS. Its just not supported by Apple, so they won't help you. That's as much as Apple is saying about it.
Its up to you, wether you want to go ahead with it or not.