Well, MGM doesn't charge parking at Circus Circus (thats still MGM, right ?). I think in the short term its all about geography. There's plenty of open space and free parking on the North End of the strip. Hell, you can just park at Walgreens, MCDonalds, or the Peppermill. So it makes sense they are the first to fall.
I definitely hope the others follow suit. For Mid strip visits I've been parking at Tuscany and walking all the way down from there. And I'll make that hike every time before I pay one dollar to a casino to park.