I haven't been at Bellagio, Mirage, Aria, Caesars, MGM, Excalibur, Mandalay, Tropicana for more than a year or more now. When they implemented the parking fee I decided it's not worth going there any longer. Not sure if Tropicana let's you park for free but I reckon a lot of people park there just to walk across the street to MGM or Excalibur and then it's only a question of time when Tropicana management decides to charge for parking.
After all, it's a long trip to get to Vegas for many people. And because there are casinos in most countries in the world people do have options. It's not necessary to squeeze out every single penny from the tourists as it may turn out that more and more tourists will go and spend their money elsewhere.
Same applies to that ridiculous 5 dollar parking fee at the Premium Outlet North. Be advised that I haven't gone there since they came up with this stupid idea to charge for parking to paying guests. Thank you very much, Vegas !!!!