They have the SD feed, but contracts rule the land of HD broadcast rights. Its a 50/50 when it comes to getting things approved or not. It is very possible that the cost to the rights to the HD were too high compared to the usual amount of viewers that purchase the standard PPV. If the SD PPV is low they sure wont spend to get the HD seeing that sell would be even fewer.
I think it's crazy not to offer this in HD, I love watching the fights on HDnet through the WEC. If the WEC is doing HD the the UFC certainly should be as it's a much bigger company. Hopefully these pay per view events will soon be in HD at $40 a pop they certainly should be.