A note in the AIRINC operating handbook about this subject perked my interest. I read it awhile back. Of course, it came up in conversation and I couldn't remember where I had read about it.
Finding the regulation took a bit of searching. It was at least a good exercise in online information research. I found the FCC regulation that stipulates the prohibition against HF usage in domestic airspace, except Alaska. I suppose it makes sense to restrict HF communications domestically, because VHF is readily available and won't cause interference with HF frequencies.
87.263 Frequencies
(b) Domestic HF service. (1) Regular
use of high frequencies for aeronautical
enroute or any aeronautical mobile (R)
communications in the domestic service
within the continental United
States (excluding Alaska) will not be
authorized.