It's true that people are just going to be much more likely to drive places if parking is free and abundant, but it's also true that places where parking is free an abundant are generally horrible places to walk. Parking, especially large surface lots in front of retail areas, makes walking undesirable.
...anyway, to be clear, I really don't come at this from the perspective of wanting everyone to walk more. I come at it from the perspective of wanting policies which reduce the need for cars, along with the belief that even though people are still going to use their cars a lot, some more people other than me might want affordable options which let them take the SUPERTRAIN to work, stumble home from a pub, not be full time bus drivers for their kids, etc...
