The FAA still requires ashtrays in bathrooms interestingly. To avoid those who do smoke there using the trash and causing a fire:

  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

https://www.law.cornell.edu/cfr/text/14/25.853