They have not released the new Shadowcoin wallet code yet, but find nothing wrong with porting their code. Since they are using the MIT license, it is perfectly legal/okay/acceptable to use their code in any way. Most alt-coins are > 80% bitcoin code with pieces of other alts. I think this is great for the evolution of crypto software.
Unlike Sling, Shadowcoin has not released a working decentralized market yet. Maybe they copied the Sling code to implement their marketplace.