The high end laptops can be setup to do this, but usually aren't as they are usually engineering laptops and the feature was usually turned off in case it was being used for something in the field. the entry level, and mid level, are setup already to low power the usb ports while on battery.
This was explained to me in orientation when i worked for Dell.
These are out of the box settings with the Operating systems already installed. you should be able to change this in power settings of the bios, or operating system. As a side note, Sony, HP, and Gateway did this as common practice as well, so i'm sure a few others would do the same.