Why parallel resistor for oscillator?
The datasheet of the W65C51S doesn't provide enough details to be sure but my guess is that the crystal oscillator circuit inside the IC doesn't have a 1 M ohm feedback resistor.

This is a typical example of a commonly used crystal oscillator circuit:



This is a Pierce oscillator circuit.

My guess is that on the W65C51S the feedback resistor \$R_f\$ isn't implemented.
It might have been impossible or too costly to do so. So an external resistor is needed.

The function of the resistor is to DC bias the inverter such that it has a high gain (large amplification) which is needed to make the oscillator work.