And, they can more easily steal data than any other person on the outside. The important point is that there are many ways evil folks can wipe you out. I'm sure you are thinking of several evil users right now. You don't want to be totally paranoid, but prevention and a bit of intelligence can go a long way. Like everything else in the world, there is usually evil lurking beyond the next turn.
Most companies seek to have customer loyalty.
They want users to pay close attention to their work, their site, their moves. Customer loyalty always yields profit and repeat sales. Some users will be loyal to you so that they can crush you. So that they can make you feel small. So they can leak bad news to the press about your operations.