People can even make money. They can sell the images. Think of pictures of crowded City streets that get published. They do not have permission from the 100s of people in focus on those pictures.
Generally where permissions come in are when you're attaching a image to something specifically. In other words, I can't snap your picture on a sidewalk and then use it as a promotional for my toothpaste brand.
But just selling the image? Generally yes even that is fine.
-Jake