If Droidscript required unknown sources to do anything (not just APK exports), then other apps could check the unknown sources policy on the device and disable certain intents (which they may do anyway at the moment, since that would mean that the applications installed may be untrustworthy). But this way there isn't any way to tell.
This is a poor assumption to make. Any data coming into your application should be assumed to be malicious. This would be the same as a server just accepting any data made to its API calls without any validation.
Please don't equate trust with any app store like that. Firstly, many incidents have shown that this blanket trust isn't warranted, and second, the final arbiter of trust is the owner of the device, not the owner of the app store.