I believe it's often times at the discretion of the FA/Purser.
My personal thought is that the FA was simply being lazy and it should be reported to UA/UAX. It would be a completely different story if the Captain had said that there would not be beverage service due to turbulence, etc.
Good for you for challenging her! I know comparisons with EU and Asian carriers can get tiresome, but I've never not had a complete beverage service on LH while flying FRA-MUC (about 30-odd minutes in the air)