The add runtime behavior has changed over the different poser versions. In previous versions it created a library folder directly inside the selected folder if there was no runtime folder (with a library folder) inside, that's not good for search behavior, when all references start with "runtime:" ;-). On Macs it even had problems when you selected the runtime folder rather than the parent folder (like not finding textures anymore...).

So it's a big improvement that a runtime folder and a libraries folder inside is now required (that's since Poser 7 or 8).