I an also confused by this as I am not sure if it is a bug, or a partial bug, or what... sometimes they ask for money to join the war, but most of the time they offer it, sometimes big amounts... I don't know if my memory is playing me, but I seem to remember that when they want to pay to join, it's usually when they hate the future enemy, and the other way around when they like them... but I don't know, at first impression it seems counter intuitive and a bug...