I have no desire for this giver/taker credo to be justified to me, so please don't bother. However, I would say that the example given is overly simplistic, almost childishly so, and ignores many critical details, such as how Alice acquired land and an apple tree, why Bob doesn't have land or an apple tree, how Bob got a dollar, etc. Not to mention that everyone needs to eat food, including Alice. (It's unlikely that Alice could survive on an all-apple diet.) Thus, if buying food makes you a "taker", then we're almost all takers. Certainly the wealthiest people in the world aren't farmers, and I'm pretty sure they don't grow their own food either.

It's a simple explanation and not a justification. From your comment I figured you don't seem to understand but it appears you don't want to accept it, that's fine too.