Not sure what that was all about other than "busy work" because the points you brought are are sound to me. Or perhaps they had to make their quota and thought you an easy target. Perhaps it was a dare or a bet.
I probably would not have taken the cup back.
"You can keep it."
They can't fine you or kick out of the airport for that.
Well they can TRY but nothing they can spin will make sense and will be unsuccessful. You complied and surrender the beverage and did not hinder their them....