Seriously? She loves animals, learned that all birds hatch from eggs and the thought of eggs being cooked or eaten in any way probably somehow disturbs her, hence she becomes a vegetarian? Is that enough of an answer?
Correction: Fluttershy loves *nature*, even the more grisly aspects of it. I don't know where this idea of Fluttershy being a vegan keeps coming from, but she clearly has a strong affinity for carnivores.