But maybe a more technical reason that I though was because...
In Youtube, when you upload, the video frames of the video being uploaded are sent to the main server of Youtube. The server then compares any frames to any other frames of copyright videos.
Example, you're uploading a Barney video which is mirrored. Originally, Barney is on the right, now Barney is on the left. While uploading, the video frames are being sent to the server, looking for anything exactly where Barney is on the left, surely they won't find anything because the real video is with Barney is on the right. So it easily passes.