pre-tenure and then after tenure it is much harder to fire the person.
At most schools having tenure means that you are no longer reviewed for your teaching and scholarship and you can't be fired except on very narrow grounds. Some schools, like BYU, have instituted a "post-tenure" review where faculty continue to be evaluated on their teaching and scholarship and could be put on warning and then fired if those don't improve.
I agree that it would be hard for someone to carry rank from BYU_I to another institution if they were to be hired in the first place.