Actually the plug could be decoding the peak/off peak hours signal (on top of the 50-60Hz mains)...It just needs a small microcontroller...
...But It can also be a complete scam (with a random powered led...) !
Please send one to clive ! 
I also wonder, how many people in a household, are actually going to bother to take notice of the colour changes, on the LED.
If you want something done, such as switching on a radio, charging your mobile phone, shaving, hairdryer, PC use, etc. A person is rather likely to just want to get it done and not want to wait around for this tiny LED light, to decide to turn green, maybe, later that day.
I suppose what is really needed, is the device itself, such as a dishwasher or washing machine. To have an option, which when on (set), doesn't start the units wash cycle, until the cheapest electricity units (usually off-peak), are available.
The button/feature would be called something like "Delay start, until low cost, GREEN Electricity, is available" - ECONO WASH CYCLES.
I suspect the success rate, of a small red/green LED, showing the best times (Green electricity, off-peak, etc), to switch on, would only be successful, perhaps 10% or 20% of the time.
But, it would probably be close to that way, without all the smarty pants mains sockets and stuff, through sheer timing luck, anyway.
I.e. Sometimes people would switch something on during peak-time, and others off-peak, just through sheer coincidences.
So the actual possible savings, could be rather minute, at best. Unless it is an extremely green/eco orientated person/family.