Mac OS X mistakes and malfeatures
We have no way to verify that there is no backdoor in Mac OS X that could install changes without permission, but that is no basis to claim there is one. I apologize for repeating a criticism of Mac OS which I cannot substantiate and must presume is false.
While Apple has not, it seems, imposed changes by force, it has a record of making users install harmful changes on pain of losing functionality, and misleading users about what these changes do.
In 2005, Apple made users install version 4.7 of iTunes in order to continue using the iTunes music store. This "upgrade" was billed by Apple as fixing a "security hole." What the update actually did was change the iTunes system of Digital Restrictions Management (DRM) to make PyMusique stop working. PyMusique was free software that allowed GNU/Linux users to access the iTunes store. (See http://news.xinhuanet.com/english/2005-03/22/content_2728356.htm and http://www.theregister.co.uk/2005/03/22/apple_blocks_pymusique/.)
Apple similarly imposed other incompatible iTunes changes later in 2005, and in 2006: users could not play music purchased using newer versions of iTunes in older versions of iTunes. So users had to update iTunes on all of their computers that they wanted to play their own music on, not just on the computer that they used to actually purchase the DRM-afflicted music. (See http://en.wikipedia.org/wiki/FairPlay.)
In 2008, Apple snuck a new DRM malfeature into Quicktime in an update advertised as adding a feature for renting movies. This malfeature stopped users from playing video files they themselves had made. (See http://www.theregister.co.uk/2008/01/26/quicktime_drm_cripples_adobe_programs/.)
If Mac OS X does not have a backdoor to forcibly install changes, that does not make it ethical. It has other malicious features, such as Digital Restrictions Management (see http://defectivebydesign.org/apple). What makes those malfeatures possible is that users can't remove them. Mac OS is proprietary software, so the users don't have control over it -- rather, the developer has sole control over the program, and employs it as an instrument of control over the users. So I don't withdraw my condemnation of Mac OS. But I do withdraw the claim that it has a known backdoor.