Why do fast food restraunts use rain foresests to graze cattle?

Answer 1

They don't. Fast food restaurants don't own cattle. Nor do many of them get beef that comes from areas where cattle have been grazed where rainforests once stood. Many franchises get their beef from locations that aren't even near a rainforest, but from ranches or farms that have raised cattle on native grasslands that have never seen a rainforest on that land for millions of years.

Now if a fast-food franchise were to get beef that was raised in South America, it was because it was cheaper to import such beef, or because it was close by (if those fast-food restaurants happen to be close to such beef operations, like those in various cities in Brazil for instance.)