Games : Landing council members

Why is it, when I decide to land a council member in order to boost their opinion, do I end up losing them? If I give, say, my spymaster a minor barony that I own, they are still my direct vassal, but I end up losing them, which defeats the whole point of landing them in the first place. I can appoint, say, a mayor as spymaster, so why can't I make my spymaster a mayor? What exactly are the rules here?