Intamin Fan said:
The parks (and not just Six Flags) should realize that money spent on parking is money that potentially won't be spent on merchandise and food.
Of course, a number of people enter the park and don't buy any merchandise or spend a lot on food; by charging a parking fee, parks and other venues have made money on each guest before they've even walked in the gate. It's all about profit, friends, and that's what these companies are in business to make.
If you don't want to pay the fee, depending on your circumstances you can take hotel shuttles, have friends or family drop you off and pick you up, park where parking is free and walk to the gate... if that's too inconvenient, then you have to pay the price for the convenience of parking close to the park.