You are right, it was petty, which is why I'm not reporting it to WHQ (which the FA clearly feared by the way she acted, so I'm guessing it would be taken more seriously than I might have thought). The issue here isn't the can, it's consistency. United's single biggest flaw, in my opinion, is lack of consistency. I had the best flight attendant I've ever had on my SFO-EWR redeye a few nights back, and as I told her, "If every flight attendant had the same amazing attitude as you there'd be no reason to fly Singapore Airlines anymore," which, while an exaggeration, gets at the point.
Then I've had absolutely awful service some days. In international business I've witnessed an FA giving meal priority to an NRSA (non-revenue, space available) passenger over a 1K (fortunately not me).
I was also once on Ted in 11C and there was a uniformed FA in the window seat, 11A. The working FA asks the FA in 11A "Well let me know if there's anything I can get you, do you want a pre-departure drink?" Saying this right in front of me was rude as well, in my opinion.
While a meal choice, a drink, etc., might be relatively minor, it's the overall mentality that I have a problem with. Some employees really see nothing wrong with treating their friends (who happen to be flying for next to nothing, compared to a full fare business passenger, for example) better than their "real" customers right in front of them. I have no problem with FA's treating employees better in terms of talking to them, etc., but as soon as it takes away from the experience of a revenue passenger there's a problem.
So it's not about the can, it's about the overall mentality of "I'll take care of my friends and screw the customers."