This isn’t the 1970s anymore, higher standards are expected and we are under much greater scrutiny than before. With social media, any in flight antics can be all over the internet before you land at your destination.
Crew behaviour down route can leave a lot to be desired. Noisy, drunken room parties disturbing other hotel guests, even incidents of letting off fire extinguishers in corridors have come to light.
Imagine the reaction if a South African Airways pilot was found to have consumed illegal drugs before operating a flight departing from London and the airline had paxed him home before the authorities could get involved.
AFAIK taking class A drugs is an offence in SA, and BA could find themselves in trouble if local laws weren’t followed when dealing with a case like this.