In fact, you can boot MINIX with a USB stick, you just can't use the USB stick.
The x86_usbimage.sh produces a bootable USB image. The bootloader will load a huge MFS image from the stick as a ramdisk and MINIX will use it as a root filesystem. The catch is that once MINIX is booted, the USB stick is off-limits.
This process uses a LOT of memory (you won't boot with less than 512 MiB of RAM) and it takes at least several minutes to load the ramdisk, but it works. It's the same trick used by the PXE image generated by x86_ramimage.sh.
By default those scripts will only unpack the minix-base set and they will not bundle the sets for installation. In theory it should be possible to work around that and perform an installation by hand, but it won't be as easy as setup.sh...