The air from the lavs generally doesn't recirculate into the cabin (thank God!), so unless you go into the lavatory after a smoker, you probably won't smell it.

As I mentioned in the other thread, (1) some people might rationalize that some actions, like putting a plastic bag over the smoke detector, don't count as "tampering", as it doesn't affect the detector's internal works, and (2) redundancy serves to overcome the fact that most people are lousy listeners. If an instruction is repeated in three different ways, it's more likely to sink in.