I started this thread out of curiousity, not a workflow problem. I've never seen this issue as anywhere near critical or important. I was merely curious because there has obviously been some change between Photoshop CS6 and Photoshop CC where the icon previews are concerned.
I figured it out, at least on my system....
I generally turn off Finder preview icons because I don't really need them and it's often faster to be able to discern .ai, .indd, .eps, etc. files quickly by their generic icon rather than having to take that extra second to check a file suffix or remember file names.
In the past Photoshop has completely ignored the Finder's icon preview setting resulting in icon previews for .psd, .tif, etc files all the time (unless that pref option was disabled or save for web was used). It made no difference whether the Finder's icon preview setting was on or off, all Photoshop files had icon previews. It would seem that with Photoshop CC, the Finder's icon preview setting is honored rather than ignored.
If I choose View > View Options in the Finder and tick the "Show Icon Preview" option, all Photoshop CC files suddenly have icon previews just like Photoshop CS6 files. If I untick the "Show Icon Preview" option, Photoshop CC files go back to generic file icons while Photoshop CS6 files continue to show icon previews.
So.... it may not be a bug at all. At least it doesn't appear to be unexpected behavior here. If anything Photshop CS6 is displaying the unexpected behavior, not Photoshop CC.
While it may be nice to be able to tell Photoshop files to ignore the MacOS setting, I can certainly live with the fact that Photoshop CC file icons just do what the OS tells them to do.