I've noticed there are certain apps which seem to track that I'm using my iOS device, and then send me push notifications based on my usage. For example, I may pick up my phone in the middle of the night, unlock it, check my email, and minutes later, this app will send me a push notification attempting to sell me something. Is an actually app permitted to track my activity/usage on my iPhone, outside of my activity and usage within that app? If so, where can I learn more about this? Or is this app in violation of some rule in the ToU?