Are ITC checks relevant and legal by car rental companies?

Very dissapointed with the service of this company! I rented a vehicle and used my credit card to pay online. On top of that I was not using credit as the card had been topped up. When I get to the Durban branch at 16h00 they tell me I'm not ITC clear and can't give me a vehicle. At this moment I had already booked a getaway for my birthday weekend, and activities as well. Where was I going to find another rental car at that time, let alone the money for it since the refund would take a while? Why do ITC check if I have paid in full. Is this even legal? Pathetic excuse to an issue with their system if you ask me.