When modern drivers decelerate motors, the extra energy gets returned back into the power supply. With an unregulated power supply, the energy gets absorbed by the smoothing capacitor(s), which under even hard deceleration of your typical machine during normal use, might lead to a few volts increase.

If you add a fuse, and that fuse blows for any reason while the motor is moving, there is now nothing to absorb the return energy, which leads to a voltage spike on the drivers input, which can be more than enough to cause the drive to go overvoltage and release magic smoke.

Plus fuses are not likely to protect the drivers. By the time enough current has flown to blow a fuse, the chances are the electronics in the drive have already failed. Fuse blowing times are measured in milliseconds upwards. Electronic failure will happen in microseconds.

Fuse selection is generally best to see what manufacturers recommend. For anything with a high start up surge, you really want to use time delay fuses.