Yeah no one should be hurt, wet or out of pocket. The empty cup is an easy and anonymous (so no confrontation) way of letting the person know they're being inconsiderate.
I remember seeing Jet play at Leeds festival years ago. Some girl in front of us doing this, until a pint flew across the crowd and hit her lower back, splashing upward. Couldn't have got more of that liquid on her if you'd tried. We were close enough to get a bit of the spray but it was worth it to hear her as she looked down at her partner and murmured "that was warm..."