Sorry, but I think you’re mixing apples and oranges here. While you are correct that the bartenders are not covered in the DSC, they are included in the gratuity you pay for the drink package, if you have one. If you don’t have a package, the gratuity is added for each drink purchased. This is different from, and in addition to, the DSC.
I’m certainly not going to argue against leaving additional, but you are mixing apples and oranges.