Well, for one, you can't drive to Hawaii, so there really wouldn't be any need to have Hawaii maps in addition to the continental US. I don't know if thats the reason for the limitation, but it's the most logical explanation I can come up with.
The same situation exists for Alaska, which I don't understand, either. In fact, that makes even less sense because you can drive to Alaska through the Canadian provinces.
What you can do is rename the maps folder and generate the maps for Hawaii. When you get back, simply delete the Hawaii maps and put your continental US maps back in place. That's a lot faster than re-installing the maps, to be certain.