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.
“No one is screenshotting…”
You underestimate what people will do to get pirated video.
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.