Guess a lot of the manners Olde England were brought to New England. Peeking into windows is greatly frowned upon ... whether you are just walking by or about to knock on the door and want to check for some reason ... the one exception is at Christmas time when people place their trees in window and leave the curtains drawn for all and sundry to enjoy.
Those who have homes fronting directly on the street usually keep front curtains/blinds drawn. And the friends-use-back-door tradition holds here.