Security Sandbox Violation?

Ok. Trying to load a swf and manipulate it’s contents:

 public class Main extends MovieClip
  private var splashScreenStage:SplashScreen;
  public function Main():void
   var request:URLRequest = new URLRequest("assets/splash.swf");
   var context:LoaderContext = new LoaderContext(false, ApplicationDomain.currentDomain);
   var loader:Loader = new Loader();
            loader.contentLoaderInfo.addEventListener(Event.COMPLETE, splashLoaderComplete);
            loader.load(request, context);
  private function splashLoaderComplete(evt:Event):void{
   splashScreenStage = as SplashScreen;

But I get a security error. Funny thing though, if I place the swf I want to load (splash.swf) into the root directory, everything loads fine.

So here’s the environment I’m authoring in:

Flash Develop 3 Beta 2
Actionscript Project
compiling with Flex 2.01 SDK

I just don’t understand the sandbox violation stuff. Never delved into it until now. Could someone offer some direction?