Think of eject for usb drives the same way as with CD's being ejected.
If you eject a CD it unmounts it and opens the CD draw or pushes it out physically. So you will have to physically put it back in to read it again. If you unmount a CD the CD stays in the CD drive, ready to be mounted again.
If usb ports could physically push out devices, when you eject the usb drive the plug/cord/pendrive would fall out the port. So again you would have to physically plug it back in again to read it.
So eject for a CD and usb are logically the same but physically usb cant eject the cord/plug/drive.
clear as mud!!!