Yuck! I never use testers, because I can just imagine the people that had their grubby little hands in them. The most I've ever done is swatched an eyeshadow here or there.
Why in the world do people do that? Especially when there are testers available! I even hate this at the drugstore and there *aren't* testers available there. I just want everything shrink wrapped so I can always tell if things have been opened/played with! I bought a lip pencil once from Sephora, didn't open it - it was still in the box, got it home and it had been USED! Now I take everything to the desk and say "I'm just going to quick look at this" in front of the cashier (unless there's a line) or order things online because... gross.
eta: I only open things I'm for sure buying