That's the way it was represented in "Notting Hill", but the version I heard is very different.

Apparently, it's ok to eat the fruit of plants - as you don't kill the plant by taking the fruit. So, since picking an apple off a tree doesn't hurt the tree, you're welcome to it.

Things like wheat, potatoes or carrots are well out, tough - because in order to eat them one must destroy the whole plant.