If I see a deer munching on a particular plant, is that enough to indicate that it's safe to eat? How about other animals?
For one, you may not be capable of gathering any worthwhile nutrition. You could eat grass all day and still be hungry.
Secondly, many animals are immune to toxins that are nasty to us. Best example I know of is that goats and many other new world animals can eat poison ivy. But if you try eating it, you're in trouble.