If it was FREE parking, it would probably be full by 7 am. How would you suggest that these people will be kept out?
By, as I have said twice previously, by only having free parking for Season Ticket holders and those who spend more than, say, ^10 (or more) in fares.
Most stations in the West Midlands have free parking. (ok, it's because they are sponsored by NWM/Centro) As a result, people drive from places outside the midlands to park there.