See, bunny poop is one of my dog's favourite things to find! I have a bichon/shih tzu
mix, but I also have 5 bunnies. It's definitely a normal behaviour for dogs. We highly discourage it because quite frankly, we find it disgusting, even though as ginger899 pointed out, its just grass and other plantes (in our case, hay and pellets).
I don't see it as being a health problem. As long as your dog is routinely checked by the vet.