Should employees have the right to refuse service to rude customers?

So today i saw a cashier get screamed at for something that was out of her control. When another customer asked them why they were being so rude, they replied “we’re the customers we’re allowed to be rude”. Then when the manager came out he told the cashier to give them what they wanted (which was a box of Halloween candy for 6.99 instead of 14.99 because they read the wrong sign). I dont care if they are paying customers or not. She should have been able to completely refuse service based off how horrible these 2 grown ass adults were acting