How does a burden resistor work?
The piezo can generate very large voltages (e.g. 90V) above the operating range (6V) of the ATmega and would blow any input. That said the power is very little and any resistance will load the spike, so that current is absorbed in the 1Mg ohm resister, rather then the hi-impedance input of the ATmega.  

Here is a link to 
Electrical Over-Stress and Electrostatic Discharge Protection of ICs by IT E2E group. It has link to a PDF presentation that does a great job explaining how the chips internals can tolerate or fail such voltages.