screen goes black when I try to mirror Netflix thru Apple TV

I can easily mirror all sorts of video from my Macbookpro to my TV screen using Screen Mirroring over Wi-Fi. But as soon as I try to mirror a video stream from Netflix (or comcast), the video goes black. (The audio and the subtitles are okay, but the video goes black.) The same thing happens with some COMCAST broadcasts, but not all. Is this a technical problem or are Netflix & Comcast deliberately blocking my sharing thru Apple TV? Same thing happens if I try using my ROKU.