It's kind of offensive when you're holding the door open for an adult, and when they walk out the door they put their hand on the door, like they think you'll just all of a sudden let go, amirite?

Actually they put their hand on the door so they can hold it for you to walk through. You hold it for them so their doing the favor back.

It's a habit.

or when they just walk right on through and don't say thank you.