Given that people posting on youtube tend to post the majority of the story, for free, it takes away from the writer’s audience, as people will then just play it on YouTube. Furthermore, the YouTube channel hits go up, and so if they end up reaching a certain number of followers and views (however it works) those videos will earn money for the person uploading them, meaning they are effectively earning money from someone else’s work and screwing over the writer in the process.
As for the splash you’re recommending to add… It’s unlikely to make any difference, as some people don’t read splashes, or simply don’t care about what it says.
Most of all though, nobody has the ability to ban anyone from episode, other than the app owners. So there is literally nothing that the writer can do to enforce the rule.