Yeah, the whole "our bodies will become grass" makes no sense.
Consider it a simplified explanation of how we get fertile soil, naturally. Two things produce good soil: Inorganic material originating from rock, and organic material that's the remnants of animals and plants. So, fertile soil that provides nutrients and nitrogen for plants, not to mention the ability to hold water, comes largely from dead animals.
The plants grow, animals eat them, other animals eat those animals, and still others eat both the plants and animals. The animals die, and ultimately feed the plants. Perfect circle.