Droids don't drink. Just take up space paying customers could be filling.
That seems to make sense, but the Mos Eisley bartender says, "we don't serve their kind," which implies both prejudice and that the droids would order something if the bartender wouldn't refuse. What they would order, I have no idea.
Although many Earth bartenders judge patrons who want to charge their phones behind the bar, so maybe it's that.