He's not, he played the festival once, and got fed up.
They asked QOTSA to play again and he said no.
He mentioned it in an interview, Sharon didn't like it, and now this.
From april 2007:

SHARON OSBOURNE has blasted Queens of The Stone Age frontman JOSH HOMME after he criticised the 2007 free Ozzfest tour for lining the pocket of the headliner and no one else. Speaking to Blender.com, Homme revealed he was approached to perform at the touring festival but declined the offer, stating, "I won't be playing Ozzfest again - they treat the bands like shit." Homme went on to criticise this year's (2007) Ozzfest, where bands are being asked to perform for free. He says, "This year you get to play for free under the guise of, 'We're doing it for the fans!' But it's really for the people who fan Sharon and Ozzy with palm fronds at their house."

Outraged Osbourne has wasted no time in firing back, telling the website, "I hope he gets syphilis and dies. I hope his dick fuckin' falls off so his mother can eat it."

I have to agree with Josh here, if they get the bands for free, why "charge" the fans?
Where do the profits go?
From the Post-Gazette:

Getting the tickets online required a passcode, which meant buying Ozzy's nearly good new record "Black Rain" or going to sponsor sites like Monster Energy Drink or Hustler Lingerie and buying some of their stuff. On top of that, Live Nation is slapping fans with $25 to $35 parking fee, which is almost the usual price of a ticket. There also are some packages in the $300 range if you want to be part of the privileged Ozzfest VIPs.

The semi-free ticket means fans get a lineup of bands that mostly agreed to play Ozzfest for nothing. That translates into a major promotion for a band like Lamb of God into the No. 2 spot and pumped-up mainstage status for the likes of Static-X and Lordi.

What Sharon said about him is very sick and un-called for.

wtf has she got to complain about?

I really appreciate him standing up against this. I've never liked Sharon. I find her annoying, overly into herself and quite frankly a little dumb. Josh had a legitimate argument that he expressed eloquently and her response was immature, childish and almost laughable. Her response ever legitimatised what he was saying even more.

Thats possibly the second most favourite thing he's said. The first being calling courtney love a troll.