They're not as similar as you'd think. The games use SSL to verify that they are connecting with the real Nintendo servers for mystery gift downloads (as well as most other online functions). For whatever reason, they don't use SSL with the GTS, and faking the server is as easy as redirecting traffic and running the right software.
In order to make a fake mystery gift server we would have to get a certificate that can pass whatever checks the games perform before finalizing the connection. Certificates can be made easily enough, but I'm not sure if it's possible to make one that can actually pass the tests.