I agree theft is wrong, and certainly do not agree with it.

But taking something that is presented as free of charge, such as crisps in a lounge isn't really theft, is it?

I do agree with people taking snacks for their flight from the lounge. The lounge's supply is with effect unlimited, but onboard there are only x amount crisp packets or soda cans. If you're particularly hungry or thirsty, by all means, take some food/drink with you!