We do all of our produce shopping at one store, all of our dry goods shopping at another (and most of our dry goods we get in huge quantities from the bulk foods section, which means most of our dry goods trips are for less healthy things). At least 90% of our diet is vegetables, fruits, and whole grains, with a small amount of meat. The things that person must think of us if she saw us in WinCo. With two small, impressionable children, no less! Scandal!
Seriously, though, IF I notice what's in other people's baskets, I don't generally draw any conclusions from it, except to amuse myself, as previous posters have pointed out. I know I've brought some really hilarious combinations up to the register before. It's not even that the assumption is the most likely explanation, though that still wouldn't make it less rude to say out loud. If I were to try to draw a conclusion about other people's junk food quantities in their carts, there are too many possible explanations to really get anywhere (this is their monthly snack food stock-up, this is their dry goods store, they're having a party, and more).