Microsoft [and others - eg 3rd party software people such as Adobe, etc] have always considered skinners' use of their base graphics/images as an addendum to its original use - not a copyright abuse.
A Photoshop Dock Icon is simply a 'third party' extra which enhances the use of the product [via Dock access].
This is NOT ALWAYS the case.
When Apple was about to release OSX a Windowblind pre-empted them which enabled Windows users to have the appearance of the Mac OS before even the Mac users. Apple issued a cease and desist - and all the sites hosting the work complied.
The first such instance I can recall was pre Wincustomize.com ...on skinz.org....
Then again, I've been wrong before....
Yep.