Are you really sure-sure they could/can not?
AFAICR it is just a matter of USB 3.x drivers
Well, I had bad experience with Win7 PE before.
First, there are many USB 3.0 controller, not the Intel which is later much supported using NUSB3. One of my system uses Fresco Logic Controller, I have integrated this driver inside Win7PE using Make_PE3.
- Once booted on USB 2.0, the USB 3.0 port can be used as USB 3.0.
- On my 'old' system, there is no option (as on newer machine) to set in BIOS to support xHCI for program requiring it at boot time (so no way to benefit from USB 3.0 boot).
- The USB 3.0 port does not show up at boot time (when an external USB HDD 2.0/3.0 is plugged).
- On newer machine, where USB 3.0 is supported in BIOS and presented for HDD boot selection, with Win7PE I got the classical BSOD 0x7B. No time to investigate it (probably to change the startup type of driver?).
With Win8-32/64 full VHD, now no problem direct booting with USB 3;0, no more BSOD 0x7B (or equivalent screen), no tweak required! No problem of SxS, missing .Net framework, not functioning 32-bit portable applicatons... Ubuntu got this function long time before Windows: installation to USB HDD, boot anywhere.