[noggin]

We can have autoframe rate on apps with refresh rate tool on Shield TV, then Why Nvidia can't integrate it on the system itself ?    

I guess to avoid the problem Roku have had with their Streaming Stick + and system-wide frame rate switching.  Roku have disabled it for Netflix (and only Netflix) because of the autoplaying of trailers in Netflix I believe.  As you switch between shows and the background trailers auto play, if they are different frame rates, you get a display re-sync and a couple of seconds of flash-bang resync on your display, which is very annoying as you scroll up and down slowly to read synopses.  If the refresh rate is API controlled, you can avoid this.  

Ah yes that would be really annoying. Maybe only have it switch when you actually push play. 

Yep - but that means it needs to be programmed into the app and API controlled, rather than system-wide I guess.  A system-wide 'output at native rate' option wouldn't necessarily know the difference between playing a trailer and a feature.