Abusing Cashiers

Most people abuse their cashier at the local discount store or gas station etc, but why?
That is my question, why do managers allow their hard working cashiers to deal with constant harassment to be treated with less than respect when a cashier calmly explains to the customer sir I don't have the capability to fix a register mistake please just go to customer service and explain that we didn't catch it in time to fix it here.
I've had a customer today tell me she wasn't going to Customer service to fix this mistake she was going to stand right there until I fixed it and demanded that I call a CSM over.

So I calmly punched in the keys to call the CSM and in the meantime began helping another customer I coudln't do more with the first customer until a CSM arrived. The customer was so angry because I started helping other customers that she started yelling at me for ignoring her.

I calmly explained I called the CSM and she'd get there as soon as she could in the meantime helping the other customers was still required of me.

Once The CSM arrived I tried to explain this customer has a complaint she's complaining about the register charging her twice for something. etc so finally the CSM took her to where I asked her to go in the first place over to Customer Service to take care of her complaint there,

Ten minutes later as I am continuing to work dealing with an endless supply of customers the CSM returns to say that customer says you were very rude to her.

But I wasn't rude at all. If continuing to work while we waited for the manager to arrive is rude I am sorry but that was my job.

And to be brutally honest I absolutely know she was not double charged for her water there were two definite different  numbers and two different types of waters in her bags of items still the managers gave this customer a refund of over 3 dollars and let her leave with free items and declared me rude.

My question is why reward rude and abusive customers why give them free items. The CSM told me it'd be okay she was aware of that customer and she has been abusive to them in the past and my past professional attitude would speak for itself anyway but I still can't understand why the store would want to take a loss to keep this customer happy when clearly she was wrong and I had register proof.

We do put up with a lot during the course of a day and each day the stress is enormous we have good reasons for coming to work even facing abuses like this one. For me it's health care I need the insurance, I'll have to work an entire year before I am even eligible for the health care I need so badly in the meantime the cancer my doctors warn me about has to wait because I can not afford the tests I need until I can get health care. So as I bide my time until then I work in a uncaring heartless environment. Is it worth a 3 dollar savings to treat someone so badly to risk their chance to have a job that might one day prove to be the only thing that saves their life because without that job the healthcare that they might have been so desperately trying to secure would be unobtainable.

After all, all I really asked this customer was to please go to customer service because I can not give a return to them at my register she blew everything else out of proportion.

Okay I know you can bellow you can get your way but really all you are doing is throwing an immature tantrum in front of everyone else.