You haven't been allowed to smoke in airplanes for years, and yet every airplane bathroom has an ashtray: it's a Jerry Seinfeld joke that writes itself. This means the bathrooms were just never updated, right? Wrong.
As The Telegraph recently reported, the FAA requires all airplanes to be equipped with ashtrays as a matter of fire safety. The idea here is that even though smoking isn't allowed, some assclown may break the rule, and that assclown needs a safe place to put out his cigarette. Basically, that bathroom ashtray is there to keep somebody from putting the butt out anywhere flammable, or worse, tossing a butt into that blue sludge at the bottom of the toilet. We're not scientists, but that might actually open up the fourth dimension.
Ashtray rules are strict, too. In fact, if an airplane ashtray breaks, FAA guidelines state that in airplanes with multiple lavatories, the broken ashtray needs to be fixed or replaced within 10 days, if 50% of the plane's ashtrays are still operational. If fewer than 50% are operational, an ashtray needs to be fixed or replaced in three days. Some airlines seem like they don't even clean the seats every three days!