Why refuse?

The AI (like real players) looks to suck up the best troops it can. In addition, if I have Venice sharing a border with Germany and Hungary at 80-90% zeal...I do not want a crusade sloshing through and sucking up my units. Likewise with a province like Hungary (a major highway for crusades) Often I cannot or do not chose to be weak in key provinces because it is cheaper (and safer) not to be.