It's a little different than the simple theft of a TV, although yes you raise a decent point. As I've been saying to people: just a reasonable effort is all I ask.

There is a watermark on the entire video, and you can quite easily tell the voice changes from the intro to the main body (where I also mention Pluralsight a few times). A very, very simple review process would catch this.

Every video I submit to Pluralsight goes through a 3-step review (Peer, A/V, tech) and they catch any possibility of copyright infringement (though yes, things do get through).

Udemy is not doing anything this way, and have admitted it publicly (see the post, I updated it at the bottom). They flatly say that they rely on their users to tell them if something has a copyright problem.