Because DirecTV doesn't want you using any products that THEY aren't getting paid for (kind of like Nextel.. they think they're GOD!)
The "dataport" on the back of the Directv receivers is rarely compatable with anything (other than Tivo or other DVR's offered by Directv). I'm not sure if this website will have anything for you, it's primarily for problems associated with ReplayTV & DirecTV. But it will at least help explain your problem.
I'm looking at a Panasonic DMR-EH50S DVD Recorder/HD combo. I've skimmed the manual and surfed the net for a couple days now looking for a reference to controling a Direct TV recvr for timed programming. Panasonic tells how to do it with cable so I figure if it's legal for cable it should be ligal for Sat. Why does this seem to be impossible? I have an old SVHS that will control my Sat recvr but using it would mean programming twice for each show I want to record....