There is actual a reason why you should not be able to copy a video and play it.

If you're using a non-DSP-accelerated codec, beyond a certain resolution, you'll just run out of CPU. From my non-scientific tests, you can play MPEG-2 Video at DVD resolution in real-time. But if you add audio decoding, software de-interlacing, and, well, PulseAudio overhead, then you do noticeably loose frames, although it remains watchable. The Cortex-A8 is much more powerful than any other ARM processors in earlier smartphones, but it's not as fast as a PC.

Also, currently I believe only gstreamer can use the DSP. So mplayer or VLC or whatever non-gstreamer thing will have to do the decoding on the Cortex. Not only am I not aware of any DSP support plans for non-gstreamer apps, but I am not sure if the DSP arbitration allows for third party application.