Perhaps because live capturing HDV is not really reliable. It isn't stable due to the GOP format. The best way to capture is to have an IO device connected via SDI.
HDV is not a good format...live capture? no no no no no...
Take heart. I did several days of live capture from a Canon XLH1 (HDV) via firewire straight into FCP 6.0.8 on a MacBook a couple of years ago and it worked just fine. I believe that I too had to set FCP to "Non controllable device".
If I can manage it in a drunken haze, so can you.
I'm trying to remember if I even had to put the camera into record. But it can be done.