Such pacts are probably also a standard way for the Dominion to operate when expanding its territories. The less direct conquering with guns they have to do, the better: the Jem'Hadar are an "army in being", a threat that is at its greatest when it is absent. That's why they are invisible, too - they can't actually fight when invisible, but one has to fear them all the time because not seeing them is no proof that they wouldn't be listening...
Treaties, veiled threats and deliberate withholding of force - these are the two favorite weapons of the Dominion.
Exactly. Do as the Dominion 'suggests', or a visit by the Jem'Hadar. For most people, that seems an easy choice I suppose.
Agreed. It was made clear many times in DS9 that the Dominion prefers non-violent means of annexation, and resorts to open conquest only when it can't take control through veiled threats or subversion of governments - or simply inviting planets to join. After all, war is not very orderly or safe, and so runs counter to the purpose of the Dominion. The Founders aren't the Klingons - they actually seek peace (which of course they define as "everybody else under our
control and kept in check through the knowledge that we're more powerful than they are")