wouldn't taking screenshots in fact not always be possible depending on the content you are watching? For example netflix with their hardware level drm? (Although its their TV they possibly have ways around that with some custom hardware)
Eventually, a digital signal needs to be converted into pixels on a screen. Once that happens, it's trivial to also pipe those pixels into a screenshot.
It's the reason DRM for digital media is pretty dumb IMO. The most sophisticated DRM still has to be decoded at some point for the end user to enjoy it. It can always be broken with a camcorder (though quality suffers). However, there exists devices which decode the HDMI signal so doing a pure digital signals.