It’s likely not what you think.

Yes, you read me right. You’ve probably even seen them yourself. In airplane bathrooms, there is an ashtray (complete with No Smoking sticker) for the people who smoke in the bathroom, even though they shouldn’t.
When I first started bringing this up to people, I encountered the same reaction again and again. People would say, “oh, it just costs too much to replace the door or take out the ashtray”. This is absolutely not the reason, though. Allow me to quote from the Code of Federal Regulations for airworthiness:
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, except that one ashtray may serve more than one lavatory door if the ashtray can be seen readily from the cabin side of each lavatory served.
The plane can not leave the terminal if the bathrooms don’t have ashtrays. They’re non-optional. That’s an awfully strange stance to take for a vehicle with such a stringent “no smoking” policy, but it really does make a lot of sense. Back in 1973, a flight crashed and killed 123 people, and the reason for the crash was attributed to a cigarette that was improperly disposed of.
Notes