myChoize - Money deducted with false policy

I booked car from myChoize **th August at *:**pm, I cancelled the booking at *:**pm.(I was calling the help centre even before that time). Even after cancelling the booking in just *mins, they asked me to pay * day rental amount as a penalty which is not mentioned in the cancellation policy. They have not mentioned anywhere that if we are booking a car with pickup time within **hours then if we cancel it then they will deduct the penalty. My question is for what reason I am paying the penalty in this case ? The base of the policy is to protect their car from long holdings from the customers and cancelling at the last moment resulting in revenue loss and customer loss. In my case I cancelled in * mins, how come it resulted in their revenue loss but if they would have charged some nominal penalty or hourly basis penalty then they would have got another customer for the car but they were adamant about a policy that is not even there in their whole terms and conditions.