Botnets.
They don't pay for electricity and they don't care.
Also, CPU-mining botnets are a very known blight of mining pools, as they consume inordinate amounts of traffic for petty gains, and proceed to DDOS pool if kicked out.
It seems like a total waste of time to run a botnet for bitcoins, each difficulty means you need to infect more computers which means more attention.
You run botnet for other things, and just mine bitcoins as a "side-dish".
Those 'nets are most certainly not dedicated bitcoin things, they just do bitcoin mining while doing other things. Also, not all botnet owners are equally smart, you know...