As a cashier, piggy customers are a real pet peeve of mine:
I can't count the times that the custy will SEE me looking at them, and instead of handing me an item and saying "I don't want this," they will still cram it in the magazine rack, or toss it on a shelf.
When I do see it, I usually say, "I can take that for you."
Most often times, I am ignored. If I am, I take great pleasure in performing some petty revenge:
I will saunter slowly out to where they put the item, and pick it up ...and just smile at them. "Oh, sorry, I guess you didn't hear me. I guess you have to wait for me to get back to my register now."
Then I saunter slowly back, peruse the item with an expression of deep thoughtfulness, and then bend down and slowly, gently tuck it into the go back bin I have under the register.
Then I smile cheerfully, "Sorry about the wait, but I am required to keep my register area tidy. So when things are out of order, I have to clean it up. I'm sure you can understand why people who just... shove things into random places can be SUCH a pain for those who are in a hurry!"
A lot of people get red faced and quiet after that.
Only once did someone have the balls to tell me that I could have waited until I had a free moment.
I gave them a huge smile and said, "I DID have a free moment, and I could have taken the item and saved time if the item had been handed to me when I asked for it."