Why isn't fast decay mode on an h bridge damaging
In 'slow decay' mode, the current flows through the two bottom FETs, which are on, and the motor voltage is near zero. The only voltage available to reduce current in the inductance is the voltage drop on the motor resistance and the FET resistance.

In 'fast decay' mode, the current flows through the FET body diodes to the rails, and from there into the power supply capacitors, briefly ramping the supply voltage up a little (called 'supply pumping'). The supply voltage is available to reduce the inductor current.

The body diodes are rated to the same current as the FETs themselves, so that they can be used for this purpose.

Supply pumping is a potential problem that has to be managed, to avoid over-volting the supply. This means at least one of using a much higher rated H-bridge than the supply volts, having sufficient reservoir capacitance, limiting the number of fast decay events, or having an active clamp across the rails to limit the peak voltage.