Fusion drive benefits for Boot Camp?
Boot Camp Assistant partitions the HDD so that Windows can be installed. Windows does not support Core Storage, hence the partition is solely on the HDD.


  Do Macs that have a Fusion Drive support Boot Camp?
  Yes. Boot Camp Assistant creates the Windows partition on the disk drive instead of the flash drive (SSD).


Source: https://support.apple.com/kb/HT201456

Speaking of Core Storage, this is what is used to make your two disks appear as one logical volume. You can't see the individual drives' content separately. For more information, see Core Storage Wikipedia or search "Core Storage".