At first glance, my reaction was "surely it should not matter if you're booting from a hard disk or from a removable USB medium; the same bootloader signature check should still be performed".
But then -- UEFI boots from a GPT-partitioned medium; USB thumb drives are typically using the older partition table format. I've not found a reference for it yet, but perhaps in Standard Mode, one can only boot from GPT media (and thus legacy live USB sticks can only be booted in Custom Mode)?