I may be a Mrs. Walgreens, because when i see people in the drug store painting themselves with nail polish or swiping on lipstick - then putting the item back on the shelf - i do say something. As sagehen said, with some things you can't tell if someone has been into it. I don't want to catch a fungal infection from someone's nails. Plus, the CVS here allows returns within 30 days with receipt, so you don't really lose if you take something home and bring it back if it's unsuitable - i've done it.
And it isn't as if there's just one "tester". Many times i've seen ALL of a particular color had been opened and used and put back. After all, why would someone want to test an already-tested item, when they can break into a fresh one, put it on an orifice teeming with bacteria and viruses, and then put it back in the rack. Yuck!
I was in a Walgreens and a stock girl saw me open an unsealed lipstick (some brands do not seal their products), LOOK at it - in no way would i test it - and put it back. And in a very loud voice she accused me of putting it on. In no uncertain terms i pointed out that all i did was look, and that i consider it to be disgusting when people test products and put them back on the shelf (because i do feel that way). And still she kept at me. I can understand. But i think she should save her wrath for the people who really do get into the stuff.