Hmmm, I am a flight attendant, and I have never been told such a thing. We stopped doing pre departures wth glasses and use plastic now, something about safety during taxi, (which is dumb... I think turbulence inflight could cause a glass to fall and break just as easy..)

I have been told we should collect drinks before take off.. if people want to keep theirs though , I don't push it... I don't see it as a safety concern (a real one anyway).

there is at least a handful of people on each flight who bring a coffee on during boarding.. and often the crew does too! We know better than anyone how crap it tastes on board!

so this guy may have been just a little bit type A that you dealt with... I would never want to take someones coffee away from them! (I am a coffee drinker myself, obviously)... I actually had a pas. bring starbucks on board for the whole crew! That was great...