Why doesn't Rufus create a Windows installation USB that can be booted in both BIOS and EFI mode?
If you are using a Windows ISO that can be dual booted in EFI or BIOS mode, you may find that the USB created by Rufus does not preserve the dual EFI+BIOS boot feature.
Especially, the Windows 8 installation ISOs, that support both EFI and BIOS boot, will be converted to either one or the other mode, depending on the option you selected under Partition scheme and target type: If you select the first option (MBR partition scheme for BIOS or UEFI computers), the USB will be bootable in BIOS-mode only (even on UEFI systems),
and if you select any of the other options, the USB will be bootable in EFI mode only (and not bootable on a BIOS system at all).
This is done to avoid confusion, as it can be difficult for non expert users to know whether they actually booted in EFI or BIOS mode, when an USB Flash Drive can be booted in both modes, and installation is meant to be a one-off operation, targeting a very specific machine and boot mode. You probably don't want to go through a full Windows installation, only to realize that it was installed in BIOS mode, when you really wanted it installed in EFI mode.
By ensuring that only one or the other can be used for Windows installation, there is no room for error with regards to which mode that was used for installation.
Note that this does not apply for Windows To Go, and that you can also enable dual EFI+BIOS boot by using the Alt-E cheat mode (see below) with Rufus 2.0 or higher.