quite simple. although not recommended you can pull the USB drive out of the port. For a CD/DVD the software eject had to work either by button or other method or you would have to reboot.... or drag the disc icon to the trash, which will then turn into an Eject icon.
I noticed the 2011's don't have it anymore by the way, the 2010 Air's still do.
It makes sense though, why should there be an eject buton for a CD drive only but not for other removable media such as USB drives.