The horizontal navigation of a site allows a user browsing through the top level pages of that site. Site administrators can use drag and drop to reorder those pages from the horizontal navigation itself, without having to go to the administration UIs.
But when some of those top level pages are inherited from the site template which does not allow modifications or a user group, the order is predetermined and the site administrator should not be able to change it. Site administrators should only be able to drag and drop those pages that are custom to the site.
Before this fix, either non of the pages were draggable or all of them were depending on the conditions (and other bugs) because there was no way to tell the inherited pages apart from the custom ones. After this change a mechanism has bee added to differentiate them and implement what is described in the paragraph above.