Apple TV gives HDCP error when playing movies downloaded and not connected to the web.

Exactly. HDCP is copyright protection to prevent copying and is built into HDMI. Which is why this is bad code. A) once a computer is authorized it should not need authorizing every time you use it off line B) authorizing should not affect HDCP. Either Apple MacBook are HDCP compliant. It looks to me as though the programmers did not think about use off line on planes and other places where broad band is not available. It is picking up an error as it cannot check. On line and through poor cussing thus triggers the HDCP message. Doing some testing yesterday. Authorized disconnected from web and then watched on HDCP screen. Tv timed out and when I turned back on up comes HDCP error. So switched internet Wi-Fi on again, re-authorized and it started to play. There is faulty video here which needs fixing.