Correct me if I'm wrong but MI doesn't have any law allowing open carry it's a default right that has no law against it. The only law I can find regulating open carry is the one referencing the pistol free zones. That law makes an allowance for CPL holders in general, no reference to "special permits" this law has no requirements that the pistol needs to be concealed or open (may be covered in another law).
Since that law was written we have a new law that prevents a CPL holder from carrying concealed in a pistol free zone, therefor a CPL holder can carry open in said zone. Which is what the AG said in her opinion without the $5 words. Is this what the lawmakers intended, I doubt it, but it's what the law says.
That being said I would not want to carry into a school just to avoid the inevitable police encounter but it would allow open carry in bars or entertainment venues; etc