Why would the LEOs unplug the device? They'd use one of those power transfer gadgets to cutover to battery and keep it running. Same as with desktops and servers. They can just grab him, not necessarily the device.
The idea is that typically people expect a laptop to remain powered when the cord is unplugged (due to the battery), and so in a situation where the laptop is grabbed the power is cut immediately (and unexpectedly) and the hard-drive is no longer decrypted. As another commenter pointed out, a forensics team could then probably grab whatever they needed from the system memory (presuming they acted fast enough), but as I understand it the problem becomes significantly more complicated.