I was advised by a CO FA that it is a safety regulation. The example she provided is that in the event of a fire, FA's would need to be able to look out the windows and ascertain which emergency exits to use.

In a situation like that where every second counts, they don't want to have to ask pax to open the windowshades for them.

Makes sense.
(edited for a typo)

[This message has been edited by eastwest (edited 05-21-2001).]