A Strata Search (if you commissioned one) should have revealed that Policy, but it sounds very much like your Owners Corporation (O/C) has Registered a Special By-Law (SBL) to limit owners’ ability to turn the car park area into a short-term parking station for the commercial use of non-owners.
The best means for you to attempt a change to that Policy is to place an appropriately worded Motion on the Agenda for the next General Meeting of the Owners Corporation, and to seek the required 75% (minimum) of those in attendance to vote with you to have the SBL amended or repealed.
Until such time as that vote can be passed, I’m afraid that you’re stuck with that $500 quarterly fee!