How to power an electric motor
First, there is resistance in the motor coils.  But in a high-performance motor it's not enough to prevent things from burning up.

Second, when a motor is spinning it generates voltage.  This is called "back EMF" (Google it).

Third, coils have inductance.  So when you PWM the motor voltage fast enough, the current on the coils will be a result of the average voltage applied minus the back EMF.

So when you're PWM-ing a motor, you limit the coil current, either directly by sensing it, or indirectly by being careful about applying the correct voltage to it based on speed.