Google sites just got easier to use. Up to this point, a Google site designer could give permission for access to a Google site in an all or nothing fashion: The designer could enable a person to enter the or to edit the entire site - or not at all. This made design of functional Google sites difficult in many ways. 

The situation is fixed now - you can specific for a given page on a Google site who can see it, and who can edit it. See 

https://sites.google.com/support/bin/static.py?page=guide.cs&guide=1387383&topic=1387643