That's probably the case. It wouldn't take long for someone skilled in win32 programming to find a way to capture a Netflix video playback and save it in a separate file. But under winrt, which is most likely locked tight by Microsoft, it may not even be possible to capture a screen image.
Of course, winrt could have holes in it...
And that's where Netflix may have a problem. By not making available a Super-HD capable win32 Netflix application, some clever people (possibly with a mental disorder like Aspeger) will see this as a challenge to create their own application, one that could potentially circumvent Netflix's own security, allowing anyone to watch Netflix content without the need for an account.
The more barriers are set up, the more tempted some people will want to break down those barriers.