For the smoking sign, I think it could be for tourists who still smoke in hotels in their country. Additionally, it can be for the hotel's benefit as well. At checkout, if they find out that the person staying there smoked, they can charge extra for the cleaning costs. So while it might not stop those that will smoke anyways, they can protect themselves by having repercussions if the person staying there did.
Now as for laws, I think the big purpose is to deter most people from doing it. Sure it won't stop those that really want to do it, but informing law abiding citizens what they can't do still works. We can see its effectivity in Singapore. A lot of Chinese tourists like to spit. They can do it, but they will get a fine in Singapore. Same goes for smokers and bubblegum users, and other prohibited things. The government can't really stop all people from doing them, but if they catch them, there will be a fine.