Suggested explanation: jet fuel doesn't need added tetraethyl lead. But aviation fuel is standardized; it's convenient to use the same fuel for jet planes and planes with piston-engines, so that airstrips don't have to store more than one kind of fuel.
Jet fuel and avgas are kept carefully separate and are incompatible - Jet fuel has no added lead.
Most piston aircraft use avgas (usually leaded) though some use jet fuel (diesel powered). Larger propellers are usually powered by turbines burning jet fuel.
