This caught me by surprise the last time I was in Vegas for an event although, as it turned out, my rate included parking. I don't know enough about Las Vegas dynamics to know all the rationale behind this. I do know that gaming has dropped percentage-wise as a revenue source.

I imagine the impact on visitors isn't that great. I know I won't rent a car unless I'm going outside of the city and it's just cheaper/easier to keep it while I'm staying on the strip. For locals, I imagine it will make some pay more and some switch to off-strip casinos but the higher gaming tiers still get comped anyway.

ADDED: I've been told that one of the issues was increased freeloading by people going away on trips and parking at strip hotels, especially Mandalay Bay but I'm not sure how much to credit that given other solutions that don't involve charging everyone for parking.