I mentioned it a couple comments down, but I'll do my best to explain.

The first reason is that we're a small craft cocktail bar, so each drink we make is made for a particular glass, like a Bicycle going in a Collins glass or a French 75 going in a coupe. Different cocktails go in different glasses because of things like ratio of alcohol to mixer, alcohol type, and type of ice used. As a staff we make our menu based on what glassware we would have available when we're at capacity and what the barbacks can handle washing, meaning that rather than have all our menu items be drinks made for coupes, they're spread out among our different types of glasses so we're using all our glassware evenly. However, if people start stealing our coupe glasses or Collins glasses, then we have to either adjust our build ratios for drinks in those glasses or stop making them altogether because different glasses have different volumes. When we're busy, rebuilding all those drinks isn't really an option, and even if I did it anyway the drink would be inconsistent, so ultimately my guests suffer because we don't have the glassware needed to make their drink the right way. It sounds like a small thing, but we win awards for our drinks and making them the right way in an integral part of that. We don't wing it or eyeball our ingredients at my bar. Picking up dishes is a very, very minor problem compared to not being able to make my guests the drink they ordered.

The more tangible thing though is that we have to replace those glasses. That's not free. Shitty shot bars might only have a few types of plastic cups that they buy for cheap, but all of our drinks are served in real cocktail glasses that we wash and dry and take good care of. Like I said, we're very successful but we're a small bar, so spending an extra $300 on glassware when I do inventory because people want to steal ours is not insignificant; that could be $300 to spend on getting our prep shed built faster or buying a new juicer to make our barbacks' jobs easier. Ultimately the issue is that while taking something from a business may seem like you're not affecting them, someone has to pay for the thing you just took, and in my case that's us.