The fuel (or lack thereof) may be free, but in the foreseeable future generation capacity and transmission infrastructure is not. Someone has to build and maintain it according to demand. If efficiency or consumption limits are not incentivized then you risk demand exceeding capacity.

Civilization grows with available energy. People would start desalinating the oceans to green deserts, speed-boat cargo around the world, grow staple foods indoors or pull carbon out of the air if energy were truly free.