Some of Newground's games like the Kirby one or the many Mario remakes are created without permission from Nintendo or whoever owns the game they're copying. Also, many of them are inappropriate. And nick.com's games aren't allowed to be hosted on other sites. They're nick-only. Same with the Adult Swim games, Cartoon Network games, and the MTV.com games, among others.
Some of the many hidden rules that crop up when you start uploading other peoples' work :p