Why is parking a bad thing? Everyone screams that people refuse to use inconvenient transit but get mad when there is an incentive to use it
Parking is a bad thing for several reasons.
1) It disincentives the use of transit to get to a train station. This creates all the normal negatives of driving from pollution, to gridlock to safety issues.
2) It results in less and poorer bus service to said train station if people are driving. You don't get more frequent service when the buses run 1/2 full.
3) GO traditionally subsidizes parking offering it mostly for free with some reserved spaces for cheap, at a very considerable cost, which means there is less money for train service and a need for higher fares to offset the cost of the parking.
4) Parking itself creates a negative physical condition around the station. Who wants to walk 500M across a large parking lot, or along the blank wall of a parking garage to get to the nearest traffic light or bus stop?
Can some parking, at some stations, sometimes be a good thing? Sure!
By and large this would be at suburban and ex-urban stations where driving is more common and where transit service is simply not going to be able to capture the majority of GO riders in the foreseeable future.
However, it still needs to be paid parking, so that, at the very least, it isn't subsidized by the train fares or eating into service-provision budgets; preferably, it would be a luxury, priced accordingly which helps subsidize the train service.
Beyond that, when charged for; there will be less parking, and it will be provided at lower cost; which again, favours transit.
There's also the matter of making sure people are easy walking distance to the station, and ideally to retail; with the ideal spot, most often, being right next to the station, where parking tends to go.
Again, this can be mitigated by building residential buildings with little to no resident parking, but commuter parking (paid) underground. Most people here wouldn't oppose that, where there was demand, and it made economic sense.
What they oppose is large parking garages that induce people to drive to a GO station, rather than walk, bike or take the bus; and the associated subsidy.