Why do people think it is rude to be quiet?

A little small talk is expected in most situations, otherwise people figure you're mad 

I guess the absolute minimum for a cashier to say is, "Did you find everything you needed?"

I don't engage cashiers in conversation, myself. For the three months I did that job, I was a little freaked out by people calling me by my name, even though it was on my badge. (Watch the earlier seasons of "Superstore," where America Ferrara wears a different nametag every day. She'd have been my hero)

I am surprised customers would say that. Most want to be done as fast as possible. One very few would expect a conversation, maybe older ones who are lonely, or those who have to talk all the time, but that can be easily solved. Just give greetings to each customer at the start and at the end of dealing with them, and in between ask if they have found everything they needed. That is all that I ever noticed other service people in your position do. It should be sufficient.

You represent the grocery store. If you want your employer to keep their customers (and keep you employed) try to be a little more friendly. Start with a nice "hello."  'have a nice day' is also good.

Just be yourself. They're the rude ones.

A little small talk is expected in most situations, otherwise people figure you're mad or something.

They say you are rude because you appear rude.

Learn to smile and say "hello" it won't kill you.

"The only time I talk is if I like the customer" if you don't talk to them how do you know if you like them?

Because they don't have anyone else to talk to. They're lonely.