Well, okay, most people are, but there's a sizeable minority of people who think that if you work in a service job (tills, waitress, etc), then you are thick scum and it's okay to take out their bad day on you.
Favourites from my time at Tesco's include:
A man losing his temper with me because I didn't look sufficiently excited that Tesco's were redesigning their uniforms;
Students impersonating me as I scan items, complete with beepy noises;
A mother from the local posh girls' school telling her daughters at my till to "work hard at your GCSEs or you'll end up working in here, like her." (I was too polite to point out that I had an A and two Bs at A Level and was just about to take my place at Leeds Uni).
These are just the funny incidents in amongst all the common and garden rudeness and weird guys leching at you (because they can't tell the difference between someone who's paid to smile and be polite and someone who actually is your friend).
So be a little patient with your checkout peeps - if they look a little surly, you might be the first nice customer after ten ignorant w*nkers!