Yes, I really hate this too. At several places that I've interviewed, either the department chair or the dean will tell me: they will offer you $X, but give you $X+5k if you negotiate. Which leaves me wondering what the hell is the point of offering $X-5k in the first place if you tell every candidate to negotiate up to $X. How is it even a negotiation if the person you will be negotiating with tells you during your interview exactly what you should ask for? Just offer that and save us all some time and angst.
But I know I am an oddball - I am very impatient with all sorts of rote meaningless social rituals, and I like these sorts of things in black and white.
This is because the chair and the dean have slightly different goals. It is beneficial for the department to signal to the college that faculty in their discipline are highly desirable, and command high salaries, this provides a justification for larger raise pools each year. The salary enhancements for initial faculty offers come from the dean, as opposed to the departmental budget, so within reason, it doesn't cost the department anything to help their new hires ask for a $5K increase in the initial salary.
