The "on your system" part is what prevents this from happening. However, with features like remote attestation, a DRM platform could be built. If users start accepting PCs locked down like tablets, then this might be an issue.
I'm not sure I follow. The DRM extensions are basically hooks to proprietary plugins. If the plugin decides that you didn't view the ad, then it will refuse to play to request media.
Then can and will be workarounds, but thanks to lobbying those workaround are criminal in many parts of the world.
At the moment, those proprietary plugins execute inside an OS and processor you "fully" control. Nothing really stops you from flipping a jump statement. (Except being more complicated than that.) I believe that's what the poster was saying: what can force him on his own system.
If you want to view the content, it will ensure that you've seen the ad before it renders the content.