Seems to be something related to Unix as a whole, or maybe it's only Microsoft Windows that turns of USB devices after unmount? Mac OS X has the same behaviour as Linux.
What if you have more than one partition? What if you want to unmount just to change mount point and remount inmediately?
Perhaps it could be better to have a "stop device" option.
As for the Windows reference up there, when you click on "Eject hardware" in Windows, you are not umounting anything, you are just stopping a device in order to remove it. You click on this or that disc or card, but the system traverses up the device tree until it finds an ejectable node and stops that, effectively stopping everything else under that branch in the tree.
Is there a progress in promotion and implementation this idea? I think, an easy opportunity for a user to power off USB drive, especially hard drive, is a must.