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.
Thord Daniel Hedengren is a designer, writer, and blogger, and also the former editor of The Blog Herald. He used to be a hotshot in the gaming industry in Sweden, but sold everything and went International. Most recently he wrote a book called Smashing WordPress: Beyond the Blog, and does loads of kickass design.