They do actually hunt for denser populations of krill. So, IMO it's closer to predator behavior than pure browsing.
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.
Also, baleen whales are not limited to krill and do eat a lot of larger fish.