When at the grocery store is it rude to consume food before you buy it?

I was at Wal-Mart the other day and I noticed someone opened up the chicken nugget bucket and was mowing them down.

To me, that is a NO NO. To me, that is rude. I was taught that you wait to eat or drink or open anything till after you've paid for it when you're at the grocery store.

What say you?