Service workers are still people.
And the world would be a much nicer, happier, friendlier place if everyone remembered that.
Corporations are soulless, blood-sucking, evil entities worthy of every ounce of scorn, contempt, and outright hatred you choose to heap upon them. This is undeniable fact. However, the minimum-wage-earning drudge you are screaming at is not the corporation. Zie is a person, who is working a job zie probably doesn't much care for, who has to put up with your bullshit or lose hir job. Zie is a person who will go home at the end of hir shift, probably tired (because my gods, not even full-day rugby tourneys compare to regular 8-hr retail shifts), and go about hir life. Zie may be a person of delicate emotional temperament, or with mental illnesses that result in emotional instability. Your temper tantrum at the register, because you find the corporation who employs that individual's policies not to your liking, may result in the clerk needing to take a "mental health moment" and quietly freak out in the bathroom. It may send hir plummeting into a downward mood spiral, affecting hir job performance the rest of the shift, and earning hir a reprimand, because despite hir best efforts, zie could barely muster the energy to cope with people after expending all hir mental/emotional spoons dealing with you.
These are things I have personally experienced, including the consequences I mentioned.* Listen, people. I am genuinely sorry that the company which is currently paying me to serve as their representative to the public has a policy that will not allow you to do what you want. I wish I could shit rainbows and make your world a better place. But no, getting angry at me will not do anything to change the return policy. It will, however, make me feel like shit, start my mental chains that end with me thinking that I'm a horrible person not fit to live, and force me to use all my emotional energy to stop the horrible depression spiral, leaving me feeling like I just want to curl up in a corner of the stockroom and cry until my shift is over, but I can't. So I drag together every last emotional spoon I have, plaster a fake smile on my face, go do the bare minimum of interacting with customers because that's all I can handle without completely breaking down, get a talking-to from my fellow managers for not being 100% "on" and being perky and bubbly with customers, and end up so demoralized and emotionally exhausted that I go home and spend my evening crying and hiding from everyone, with my very patient fiance making sure I also eat and go to bed at a reasonable hour.
TL;DR version: The temper tantrum you feel entitled to throw in the face of a service employee because you're not happy with their employer, may very well be the pebble that starts the avalanche that buries their entire day in a haze of misery and trashed mental health. You wouldn't feel entitled to abuse your average person on the street that way (and if you would/do, I suggest you seek psychiatric help and stay in your house until that changes). You are not relieved of your good-manners obligation to treat your fellow human beings with respect and dignity and fairness, just because we're on the clock at the time. We may be corporate employees, but we are first and foremost PEOPLE. Please remember that and act accordingly.
This public service announcement brought to you by some first-class assholes at work, and also this post.
*For new readers: I have depression and social anxiety. This means that my mental and emotional coping resources are often severely limited, and become taxed much easier and more rapidly than a person without my mental illnesses would experience. Yes, one nasty customer *can* throw me into a very bad spiral that ends with "Oh god I'm just going to run my car into an overpass on the drive home tonight, I can't take this anymore." This is a result of my malfunctioning brain. No, not all service employees struggle with my particular issues. But you can't tell I have these problems just by looking at me, so it's not safe to assume they don't, either.