There are different reasons for a charge point operator to raise a blocking fee:
In general, blocking fees are raised by the operators of the charge points to encourage drivers to remove their fully charged vehicle from the charging station, so another car my be charged.
Some charging stations might be located in parking lots where parking fees apply for gasoline cars but not for BEVs during charging. In order to discourage BEV drivers from using these spots as free parking spots after the car is fully charged, operators of charge points might raise blocking fees.
Lastly, High Power Charging Stations can quickly charge one car after the other and thus should not be blocked by a car that is fully charged as it would defeat their purpose.