Why would people who live five minutes a or fifteen minutes want to drop hundreds of dollars on a hotel room when they can just drive there? Yeah they may not spend as much money, but they're still spending money.
They have never marketed Volcano Bay as anything other than a water theme park. Nowhere has it ever said that it would be a resort-only park. With all the advertising, commercials, billboards, etc...this is marketed to everyone, and to not include a major demographic would be a horrible move for Universal. They have a large local passholder base.
I get your points...they make sense...but if Uni only wanted this to be a resort exclusive park, they should've marketed it that way from day one.