That is a setting in "advanced settings" under "syndication"
If your vid matches a third party content i.e. copyright, you can't change this setting.

Notice here on one of mine that was flagged for copyright, the one on the left, that they also enforced "notify subscribers"

[link to postimg.org]

The one on the right is third party content free, so I'm able to change the settings.

Copyright owners do this because they make less money with mobile clicks and that's also why they wanted me to send it to my subscribers - more people, more clicks, more money.