Wisconsin Governor Scott Walker: "We don’t have anything to give."

This is Walker's explanation for why he's trying to cut benefits and take away most collective-bargaining rights of public workers in Wisconsin:

“I’m just trying to balance my budget,” Mr. Walker said. “To those who say why didn’t I negotiate on this? I don’t have anything to negotiate with. We don’t have anything to give. Like practically every other state in the country, we’re broke. And it’s time to pay up.”

If that's true — if he has no choice because he simply doesn't "have anything to give" — then why does he want to exempt police and firefighters?