Stealing. You wouldn't take a bite out of an apple then put it back because it didn't taste good. Nor would you pluck a banana off a bunch and eat that just to see if "the rest of them are ripe". It's the same reason you wouldn't open a bag of potato chips to test one and see if it's crisp, or a slice of bread from the loaf, etc, etc...
Just because they're out in the open and accessible for you to take doesn't mean you should. If they have free samples out then of course, go ahead and try. Pulling a few off a stem, just because you can, is still wrong. Buy them, and eat them, just like you would with anything else.