Actually, some do. Previously wore one that I looted while doing my class quest on Alderaan, and equipping it would lower the hood on the robe I was wearing. The visor I'm wearing now, however, leaves the hood up. Though seeing as it has better stats and is far better looking, having the hood up is in my opinion a small price to pay.
That being said, as there's a hood-down option already coded into the robes (as equipping the proper headgear proves) then they should allow us to choose which look we prefer without mucking about with headgear.