Question about Push Notifications and Use Tracking

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?

Question about Push Notifications and Use Tracking
 
 
Q