Because whatever it is, 99 times out of 100 it probably won’t be their fault.
I’ve worked in a cafe, I’ve worked in a bar. I used to get yelled at by people a lot, about things that were completely out of my control. Because (surprisingly) the minimum wage paid barista is not calling the shots in the establishment. I used to stand and listen politely as people yelled their grievances in my ear. I was trained by my bar company to only listen and not respond, then pass the information on. I did as I was told, but it didn’t mean that it didn’t impact me emotionally at the time. No one likes being yelled at.
You’ll probably get peeved by places from time to time, but taking it out on the staff playing the face of the business is unlikely to result in anyone listening to you. In truth, you’ll most likely just get a few rolled eyes behind your back once you’ve gone. Take your gripes to the people that can do something, and are responsible – the management. Even if one of the team on the floor has upset you, still always better take it to the top, and nowadays you can reach them very quickly with the use of the internet.
It’s easy to blame the tired barista working the 8 hour shift when the prices are too high, or the coffee’s no good, or the loo’s not in the state you want to find it, but in reality the power to change it will lie much higher up – so seek out where the person that can best help, and take it there, chances are you’re more likely to be heard, and make a little more impact than having a momentary grumble.
(To add, I don’t believe it’s ever okay to yell at anyone, but that’s another blog post.)