How does the viceroy butterfly not benefit from eating milkweed?

The viceroy butterfly does not eat milkweed, it is a mimic of the monarch butterfly which does eat milkweed.

The milkweed makes the monarch butterfly toxic to birds. Once a bird eats its first monarch butterfly it gets so sick that it learns to never try to eat anything that looks like a monarch butterfly ever again.

The viceroy butterfly has evolved to mimic the monarch butterfly to avoid being eaten by birds that have previously tried eating a monarch butterfly.