Buying a region, by its very nature, circumvents various restrictions. It's not using military force through normal channels to convince people to abandon their current allegiance and accept your rule: it's bribing officials to proclaim that you were always the rightful Lord of the region, they just didn't realize it until then.
It also costs a huge amount of gold for any region worth buying. Off the top of my head, I would guess that buying Haul cost Velax in the neighbourhood of 4000-6000 family gold. That's the kind of thing that you can only do once, and if you do it in a situation where you're not going to be able to keep the region, you'd damn well better get some kind of situational benefit out of it, or all that gold is just thrown down the toilet.
If buying a region were something that anyone could do whenever they wanted, yes, it would have a lot more restrictions on it. As it stands, the power and flexibility of the option is counterbalanced by its very high cost.
