The whole charging for parking is really just to be a deterrent to "bad elements" just showing up on CityWalk. If they have to spend money to do it, they probably won't and they'll go somewhere else that isn't Universal.
I remember when I was 21 and worked in a nightclub and the manager explained that the clientele you get when it's free admission is not the clientele you want. The nights where there was no cover brought out the dregs. The nights with a cover brought the better crowds (who spent more money and tipped better).
If you REALLY want to go to Universal for HHN that badly between the hours of 10-12 and the parking fee is stopping you, I have questions. If you're that big a fan I assume you have multi-night passes or you're spending a lot of money for just a couple of hours. And if you have a mutli-night ticket I'd guess you have an annual pass that gets you complimentary parking anyway, The few people who this will effect are people they don't want or need. They're the people that are causing issues in the parking garages.