This LA Times article details the Hives / Epitaph / Warner Brothers deal and gives some insight into how it came about.

To summarize: In the mid 90s, Epitaph was a hot property with the success of the Offspring and Rancid. Brett Gurewitz turned down countless offers from most of the major labels to purchase his company, turning his back of millions of dollars in the process. One of the offers was from Tom Whalley, former Interscope head and current CEO and chairman of Warner Music. While Whalley was turned down he and Mr. Brett became friends. However the loss of the Offspring was a big blow to Epitaph, something that Whalley says Mr. Brett is trying to avoid with the Hives. "I don't think he wanted to go through that situation again," Whalley stated to the times, "And the Hives had moved into a situation where a major can put up the money and manpower and relationships that we have with radio programmers and retailers that are different than what an indie has. That's the conclusion he came to."

Technically, the Hives however will NOT be part of the Warner Brothers roster. Instead, a special arrangement has been made for "Veni Vidi Vicious" and the band's next album to be stamped with the logo of Sire Records. Sire (now owned by Warner Bros) was in the '70s, '80s and '90s home of the Ramones, Talking Heads, Smiths and others. Sire was deactivated last year in a restructuring, but Warner thought the Hives would want to represent the Ramones' old label. The joint partnership exists for the "Vicious" and the band's next album, and according to the article is a one-time deal.