Now these tribes are already hostile to "foreigners," which pretty much means anyone who isn't blood kin. And while they're quite conservative, religiously, they aren't necessarily as conservative as either the Taliban or al Qaeda would like them to be. So the pact offers an incentive to them to clamp down on al Qaeda and Taliban activity.
And it also includes provisions for the government to take action against "trouble makers" that the tribes can't handle themselves. The Pakistani government knows that simply backing off and giving al Qaeda a safe haven is suicidal. Al Qaeda has, since 2003, been making suicide attacks against Pakistani government officials, including president Musharraf.
In Pakistan, the government pact with the tribes in Northern Waziristan (along the Afghan border) has some interesting intricacies. It pledges the government to leave the tribes alone, so long as the tribes prevent their people from undertaking attacks into Afghanistan or against the government. In addition, the tribes promise to stop "foreigners" from using Pakistani soil to undertake such attacks; in this context "foreigners" is a euphemism for the Taliban and al Qaeda.