It is different because it is your content/library. Where as Game Streaming from Microsoft servers can add/remove games at any time. And rules need to be fair for any developer, not just unique to Microsoft because they are Microsoft. I can certainly understand their situation as this could lead to apps that steam other apps just to bypass Apple's review process. If it is even a semi-opened hole in the iOS walled garden, hackers will be ALL OVER IT.
Just like if you built a website that is only intended to be used for employees of the company. Do you prevent SQL injection? But you trust all the employees! Yes, you should still assume some malicious input could be entered in the text boxes. Maybe the employee was just fired and wants to cause damage.
You want to make sure your environment is locked down. I can see Apple's position here as it could lead to hacker streaming apps. Unless people DO want Apple to play favorites here.
It's not about security breach, there will be no hack attack because of it, it's not about anything like that. There is no other logic or intention behind it but Apple protection of it's own native game market and it's own gaming services. That is all there is to it. Same reason why flash was not allowed on from the day one, cause it was alternative way to apps and games.