Better to make some money from the people watching your streams on a computer without an adblocker, than it is to hide the stream and have people hit their back button and find one of the dozens of other sites.

The cost to run one of those sites is being overestimated here. They don't actually serve any of the video streams, just embedding from the dozens and dozens of little-known free live streaming services.

Any site that tried this would be replaced immediately by another that didn't.