It's tough in the endzone. If you turn your head to the QB, your assigned receiver can just run along the endzone and get free. In that situation it's best to just do your best to faceguard your guy and hope their QB can't put the ball in that spot.
On a fade route. Fairly easy to recognize when the receiver is running the fade. He doesn't have much room in the back of the end zone. On a scramble play I agree with you but on the fade route you need to find the ball and go make a play.