Flash carts are legal as long as they are used to download homebrew apps or games. If a programmer makes a product and wants to distribute it freely to the homebrew community it's fine. The problems happen when people begin using their flash carts to download retail games for free on their Nintendo DS. Basically, it's just like downloading music illegally, except its with games.
Here's an example. Most music costs money at record stores...but occasionally you will find a garage band at a small club that is willing to pass out their home pressed CD at the show for free. The homebrew community is kinda like that garage band. They don't necessarily want money more than the simple fact that they want to be heard. And there's nothing wrong with listening to free music, is there?
AlienDS is a homebrew game made by one guy who just wanted to show what he was able to do with the DS. So you can download it without a guilty conscience.