Because it looks like it's about to run away from you, and we certainly wouldn't want that to happen. Nuh-uh. It's not as if part of our job is to track every bit of skin you show.
Your reflexes may have saved you from showing nipple this time, Ms. Hudgens, but you might not always be so sharp! One day—hopefully while you're still in Hawaii—a sneaky wave will come and deftly untie your top with salty fingers. And then guess who's going to be right there? Not us. It'll be some other folk with strange careers. (We might be evil, but we're not the paparazzi!)