If you have a look at the schematic you will see that although the voltage and motor current is sourced from the 9V supply in the PET, the voltage that is fed to the motor is very close to 6V. It is a 6V motor.
The reason is the voltage is initially regulated to 7.5V by the Zener diode. Then that is passed by a Darlington transistor pair configuration wired as an emitter follower, so very close to (0.65 +0.65) volts being the Base-Emitter junction voltage drops, is subtracted from the 7.5V. So very close to 6.2V DC is applied to the motor when the PET activates it.
To activate the motor, they simply have another transistor across the 7.5v zener diode. When this transistor is switched on with base current, it effectively shorts out the 7.5V zener reference and then no voltage is applied to the motor.
Typical of many motors in economy cassette players, the motor's speed regulator is internal to it, it is not an electronic controller, in most, though I haven't physically opened a Datasette motor yet. Generally the motor's rpm is controlled by a mechanical inertial switching system which interrupts the motor current at intervals to stabilize the speed. So the motor's applied voltage (within limits) doesn't affect the RPM significantly because the internal governor compensates it. Though, with cassette units line this, if you make a recording of a 1kHz tone and play it back, you can hear the action of the governor as small pitch changes occur, corresponding to when the angular velocity of the motor shaft is speeding up (governor contacts closed) or slowing down (contacts open). In a digital recording these amount to timing errors, but they are not significant enough to upset the apple cart for the data extracted from tape. In disk drive motor controllers, they would be, so these normally have an electronic speed control board with a tacho assembly on the motor shaft. More expensive audio cassette players have this too.
( One thing about these motors too, though it is not a problem in the PET, they often have a filter capacitor directly in parallel with the motor terminals. It is to filter off switching noise which comes about because of the motor's inductance and the governor's contacts opening and closing. If a transistor is used to switch the motor on & off , like in the PET, this is no problem. However in other computer designs, one example being the SOL-20 they use a small DIL relay to do it. The capacitor's initial charging current can cause the relay contacts to stick together and the motor won't switch off. In the SOL they tried to ameliorate this by putting a resistor in series with the contacts, but that lowers the applied motor voltage. For my SOL that uses external cassette players, I internally modified the cassette units by adding a driver transistor for the motor, which protects the small DIL relays in the SOL)
