Good question.

I also would like to know although I'm only doing an ICF foundation.

we have had an odd mix of results with this issue, some within the same company like state farm and allstate. We've had 2 agents from each of the above companies classify the home as a masonry structure, and the others for some reason could or would not, with no real reason as to why. I think it had to do with lack of knowledge and therefore they were timid to classify for fear of issues down the road. My beleif is that this will have more to do with the agent themselves and not the company. Don't be afraid to change agents either. If my agent wouldn't classify an icf home correctly he is not doing his best to look out for me and I'd look elsewhere. Besides, it is a concrete home and should be classified as such. Good luck and hopefully you can find an agent that can help.

WHY? Why have the benefits of icf for only half the house?

I doubt you'd see much savings unless you found a company that specializes in writing icf homes. With a new home the replacement cost will be higher for an icf home and your premium will actually be higher than frame with most companies since there is little or no discount for masonry construction. With same year built and identical coverage there was no difference between frame and masonry construction with 3 companies I looked at.
I'm sure high risk wildfire areas would see premium savings or maybe even in tornado prone areas or hurricane areas.