FAA rules. It has a lot to do with safety. In case of a crash landing, or a mishap during takeoff the ground crews should be able to get to the right spot to help and save lives.

They need to see inside.

This rule exists because of several instances in the past during plane crashes, ground crew could not help properly. After enquiry commission recommended that such rules be implemented because the hearing in such enquiry revealed that lives could have been saved. Now it has become a FAA (Federal Aeronautics Administration) rule.

It is just the same reason why small objects should be tucked away safely (it can be a projectile during a crash), seats must be upright (to prevent injury) and belts fastened (same reason).

Also if you recall, in some instances a plane landing with the window shades up can be perceived as being hijacked. I remember several years ago landing in DFW and the FA forgot to tell us to lower the shades - it was a hot day - and two police cruisers were waiting for the plane at the gate. During the summer, once the plane is parked and the door open, here in Texas they usually ask to raise the shades back up to keep the plane cooler.