I started building Inns because I thought I would be able to hire mercenaries while I built up my regions to train my own troops. Then when I got some training facilities after awhile and started training some units, I noticed the hire mercenaries box in regions with inns.

The first campaign I played I had auto build buildings on so I could get used to a few things. I remember the AI building a bunch of inns early on. Why would it, or anyone do this if they can't be utilized until a training facilty exists?

Also, this whole system seems odd to me. You are hiring the mercenary unit, not training it. WHy else would it pop onto the map right as you hit the pay button? To me that says they already existed and you hired them. In this light you should be able to hire them from any region that has an inn regardless of whether it has training facilities for your own troops. I mean, how does a faction train a specific ethnic mercenary unit it has no ability to train in it's own facilities otherwise?