I could be wrong about this but I believe that the graphics component of the broadcom chips used in the H/HR series can only do one HD and one SD display, and since directv uses the picture in guide/list/menu all the time, if thats an HD picture then the guide/list/menu has to be in SD. It'd be ugly to sometimes render an HD interface and sometimes an SD interface depending on what show was in the little picture-in window. Might also be implausible to downrez the picture-in window to SD so as to render an HD interface.
I'm not 110% sure of this, its just something I think I saw in the broadcom specs. Dont know anything about the overlay capabilities of the chipset in the HR24. Perhaps that can do an HD/HD overlay and they'll offer the HD interface only on the HR24 and subsequent series.