You don't even have to look in the agreement. Applications are still sandboxed so while certain parts of them can run in the background, they still can't modify part of the core operating system like the lock screen.
Now technically a workaround could be where developers would use local push notifications to display the information when you set your alert, but if it works like how the current push notification system works, that popup will disappear if you get another notification from anywhere else.
Thats what I hate, I want the apps to modify the software.