Historically, yes. However, since the box is internet enabled, they could very well do it VoIP.
There's really no need to. Doing it over satellite is more efficient. Even that XStreamHD thing was supposed to do the majority of the downloading over satellite.
And doing it over IP would leave out a lot of people. DISH has high penetration in rural areas, where true broadband isn't available. I'm on Wildblue myself. I can hook an STB up to get guide data, or surf the web, or run weather widgets or what have you. Maybe even download something of my choosing. But start pulling1080p movies through that connection all day, and it gets useless pretty quick, plus I'd get FAPed withing a day or 2.
The main barrier to offering large VOD selections over satellite is storage space, not satellite bandwidth. Running it all over IP is just going to make things more complicated and clog things up.
You think people are mad about have a chunk of their HD not available to them...wait until they start putting in an extra tuner just for VOD, so they can fill that thing 24/7. Heck, it wouldn't surprise me if someone cracked this 922 open and found one.