The real problem, I think, is that Microsoft thinks requiring admin rights to write to "Program Files" is the be-all and end-all of solving the "application-environment integrity problem." That works for enterprise-wide deploys of sysadmin-supported software, but falls down for user-specific installations. On OSX, "application-environment integrity" can be enforced easily enough, since the OS delineates applications by a line called "the app bundle." OSX can (though I'm not sure it does) just disallow apps from writing into other apps' bundles without a "do you really mean it" prompt. But in Windows, the The Directory Is The Application Bundle[1], and so Windows doesn't know that this directory is special and should be protected from having other apps in other directories tinkering with it.
[1] http://blogs.msdn.com/b/oldnewthing/archive/2011/06/20/10176...