Not everyone is the same. My teeth and gums are not that good and I definitely feel much better brushing them after every meal (though I'm aware I should wait after acidic food).
(time-wise, I started increasing brushing after the situation got worse. The root cause for the worsening was not seeing a dentist for two years. Don't do that, definitely see a dentist periodically. And the dentist seems happy with my brushing decision, need to do as much as I can to prevent plaque formation)
For airplanes, I buy three small bottles of water before flight (15hr flight). And I use that for drinking, rinsing and washing my toothbrush. While we cannot bring bottled water from outside the airport to the embarking area, there are usually shops in that area that sell small bottles.
If I'm taking an 8 hour flight after hours at the airport, transit, etc. and am already using the plane bathroom for physiological reasons, I don't mind a quick brush with bottled water...
I think that's a bit of a harsh take. People will use what they can get, and they may be assuming the signage was placed there for compliance/legal box-ticking reasons rather than because it will actually make them sick.
If you fail to heed a warning, though, the law provides that you assume the risk of injury that could result and contributed to your own injury. Without assumption of risk, anyone who provides any services would be strictly liable for any injury, even for those that don’t result from inherently dangerous activities. That would mark a significant change in the law and would suddenly make a lot of activities and services infeasible to provide.
