You can update your book for any reason at Amazon, and they'll note in the Manage Your Kindle section that an updated version is available.
If the update is to correct some grievous problem, such as formatting errors, they'll send out an email to everyone who's bought it, announcing the updated version.
I can't properly express how outstanding I think their practices are in this regard!
It's possible that traditional publishers (vs. self-publishers) have fewer formatting errors and therefore less need to update, but I've seen some pretty horrible formatting come out of trad publishers. I think they probably don't obsess over any individual book the way a self-publisher does, and they have the added consideration of keeping an ebook in sync with the printed version.