I don't keep score or judge what people are buying but I do take notice of what is on the conveyor belt in front of me. The magazines don't offer enough distraction for me and only one grocery store near me has TVs in the checkout line.
Sometimes I see someone buying a large quantity of an item and wonder if there was a sale that I missed. Or I'll see an interesting item in their grocery order and make a note to look up how it is prepared when I get home.
When you're in public, everything you do is observable by others. I think that it's naive to believe that people won't notice what you're purchasing. I only consider it to be an act of rudeness if you actually make comments to the person about their grocery order. For goodness sake! What difference does it make what people think as long as they have the good sense to filter their thoughts before they reach their mouths!
It's one thing to glance at another's groceries when they are in eye view. It's quit another to take stock of all their purchases AND look to see what type of payment they are using.
I'm not naive, I know there are nosy people out there, the Dis has sure taught me that, but I also believe people should try to mind their own business as much as possible.
Just because someone doesn't know you are being rude and prying into their business, doesn't make it not rude, IMO.