Dunno about the legality of refusing to open the door, but it does sound like a way to get banned from a hotel chain.
Edit:
Sorry, that’s from the wrong point of view but I don’t think the answer changes. It seems Rest will have to change a lot of their marketing language to really avoid liability but if someone is actually caught smoking then it’s not likely to manifest.
It would be unfair to charge people with just a black box algorithm. But a few door knocks could fix that, one way or the other.
You realize there's a difference between cigarette smoke and mustard gas, right? Standing at the door of a room that someone smoked in will have no effect on your health whatsoever. You need to be standing in the room, while they're smoking, preferably for a few decades.
I wonder if they could legally separate this from any real-world activities completely? During check-in, put a clause in the contract "if our partner company says so, you have to pay $500 extra. By signing, you agree to that" - without any reference to smoking at all.
I hope this wouldn't be legal, but it sounds like it could be.