Here are a couple of links with information about 7075 and 6061. I think 6061 is used because it is cheaper than 7075 and is widely available in North America. I also think anything that can be made out of 6061 can probably be made from 7075 without any problems other than 7075 not being as corrosion resistant as 6061.
http://www.unitedaluminum.com/aaa.php
http://en.wikipedia.org/wiki/7075_aluminium_alloy