Cows also seek out denser grass, so that hardly destroys the analogy.
Also, baleen whales are not limited to krill and do eat a lot of larger fish.
Granted, I've never observed grass to see if it avoids cows either. /shrug
These defensive measures have a huge range. Some might only increase healing, others might attract life higher up in the food chain to attack the herbivores, and still others might be able to actively damage the herbivore.
My point is that eating plants may be less passive than we think. All of life has had billions of years to evolve, so the interactions can be pretty sophisticated all around.