Quiet obviously if the said crew member is seated in the cabin then he/she is NOT flying the plane and therefore I see no problem whatsoever in that person having a drink.
Aside from failing to understand your rant about white shirts, I dispute your logic Speedbrake! Travelling in uniform, they could be a crew positioning out to fly the same aircraft back for example. The pax don't know do they. So don't give them amunition. Even if just off duty, and not flying for several days I would think it prudent not to drink in uniform.