Videos

I only give my students points for topics mastered. They have found ways to cheat the system with minutes and points, but there isn't really a way to cheat with topics mastered. I let them work on topics from any grade, but they quickly run out of the lower level work and have to work on more challenging skills. I would let them know you are on to them and will not give them points for just playing videos. 

I thought that only my students to this.

Just as if you show a video in class, there is no guarantee that a student will remember anything from a video shown on Khan Academy. The student might well have their thoughts and attention on something completely different.

If you want to know if they have learned from the exposure, you need to test it, and if you want them to pay attention, for some of them it is important that they know they will be tested.