Every airline I've been on has an ashtray in the bathrooms, ironically right below the sign that says no smoking in the bathrooms.
If it's prohibited to smoke on airlines, and (in theory) you can't bring a lighter on the plane anyway, what's the point of the ashtray? If it's for cigarettes smoked before security, there are plenty of garbage cans and ashtrays outside of airports.
It's the law:
"Regardless of whether smoking is allowed in any other part of the airplane, lavatories must have self-contained, removable ashtrays located conspicuously on or near the entry side of each lavatory door"
It's because if someone does decide to break the law and smoke in the bathroom, they would rather you put your cigarette butt in the ash tray, rather than the trash can, which is a major fire hazard.
