You'd be surprised. Cuba's state airline only banned smoking in the 2010s, and Chinese pilots were allowed to smoke in the cockpit until the 2000s at least even though it was banned for passengers in the 1990s.

Also, I guarantee you (considering people try to light up in the bathroom anyway) that if they didn't say that, people would try to smoke on planes more often.

Aviation laws require the no smoking signs to be put there, and ash trays to be in the aircraft bathrooms even though smoking is of course never allowed. Sadly basically all the safety rules are because of some prior incident that cost lives. You would hope a reminder is enough but some think they know better than the rules.