Why do they do this? In their own words:
1: Technically, we have to. :)
2: It’s important to respect the spirit of WordPress. This is the way it was intended.
Most resellers of premium (commercial) WordPress themes have gone this route, if nothing else but to be included on the wordpress.org featured page is my guess. You might want to refer to our guide to the GPL license for more. You can purchase ThemeForest themes here.