Because you already raised them in the province as defense and don't have to pay them anymore after that? Don't expect that they'll give you back their payment when you think that you no longer need them. Also, they want to keep their jobs. They have a wife and kids to feed, you know?
I want to know as a newbie whether investment on pd is worthwhile in SP with impossible AI. The ai do like to throw loads of eagles and madmen to you. But at the same time, they build huge armies. So cash is always short. Shall I build more troops or spend a few bucks on PD. And how much PD is sufficient for beating madmen and eagles.