When a company makes a profit, that doesn't necessarily mean they made anyone else worse off. In general, when in a competitive environment, and dealing with customers who are responsible adults (which both hold in the case of the restaurant industry), we should presume that everyone is being made better off by the transactions, that it's a win-win situation.
Well, a union is a form of cartel, it's an anti competitive organization of market participants who are colluding to set prices and extract other concessions from labour buyers. They therefor undermine the ability of markets to maximize value for all participants.
