From what I understand, it's a bit of a misnomer, but modern technologies have gotten coal burning to the point where they can filter out about 90% of the pollutants. It's not 100% clean as the name implies, but it's a LOT better than where we've been. Obviously solar or wind (hydro, thermal, etc.) would be best as those have no emissions. Even natural gas and biomass would be better than the cleanest coal. But "clean coal" is a decent intermediary step, especially since America is about as coal-rich as the Middle East is oil-rich.