This is very different. Preventing rightclick can be done silently, without the user intervening. Mouse lock, on the other hand, must be explicitly accepted by the user.
In fact, the current implementation ties this to fullscreen, combining the prompts into one. In other words, the only way mouse lock will happen is if the user clicks on a button for fullscreen and then allows the site to use fullscreen+mouse lock. So there is very little risk of annoyance here like there is with rightclick prevention (but is still exactly what a first person shooter needs).
In fact, the current implementation ties this to fullscreen, combining the prompts into one. In other words, the only way mouse lock will happen is if the user clicks on a button for fullscreen and then allows the site to use fullscreen+mouse lock. So there is very little risk of annoyance here like there is with rightclick prevention (but is still exactly what a first person shooter needs).