You don't normally ride in the backseat when you friend picks you up. Your friend doesn't have an Uber sign in his front windsheld.
Sure, they could hide that sign and request that you sit in the backseat (I've actually had Uber drivers at LAX request this). But then they would be asking drivers to take steps to knowingly break the law.
I always ride in the backseat. It gives the driver his or her personal space, lets them use the front seat for their snack or bag or whatever, and feels more professional/businesslike to me.
Sure, they could hide that sign and request that you sit in the backseat (I've actually had Uber drivers at LAX request this). But then they would be asking drivers to take steps to knowingly break the law.