The only reason you believe that is because you do not know anything about nature. Not an insult.
Deer, if left without a predator can destroy entire ecosystem by eating the entirety of the vegetation, which reveals smaller prey, dries up river beds and kills further vegetation.
Dolphins, Sealions & some species of African insects are notoriously known for raping everything that breathes. Dolphins rape for fun. The insects rape with so much ferocity and numbers the females can end up a puddle corpse by the end of it.
Nature is a constant cycle of pruning perceived "harmless acts" before they snowball into disaster.
The deer think they are just eating, the insect thinks it's just doing its job, but they need to be kept in check
Hunger, the weather, and other animals keep them in check