Revolution Slider is a premium plugin which can be automatically updated via a purchase code not the theme purchase code, but the plugin purchase code).
You don`t have to buy it, because you earned your right to use it for free, within our theme.
However, you`re not able to make automatic updates of the plugin, because it requires a license key, which can be obtained only by purchasing it. You don`t have to, because plugin updates are managed by us, with each theme update, before ensuring that the new version is working fine with the theme.
That`s how things work.