This “feature” accomplishes nothing of value for anyone, including the streaming services, but imposes a massive (and for most people, confusing and frustrating) hindrance on honest people simply trying to easily capture high-quality (as opposed to, say, using their damn phone to take a photograph of their reflective laptop display) screenshots of the shows and movies they’re watching.
Have you ever heard of mjpeg? It’s literally just a bunch of JPEG images that are frames of a video. If you can take screenshots fast enough you’ve got yourself a video. It’s also how Blender renders videos (frame by frame).
It isn’t until later in the output pipeline that you render frames into interconnected streams that single images turn into a proper compressed/encoded, “video file”.
So yeah: Screenshotting videos is a perfectly valid way to copy them.