also by your logic if notepad has a bug that's exploitable by opening a text file, it's an a windows issue.
somehow I believe you'd be on here complaining about how bad windows is even if the problem was in notepad.
Let me fix that....
also by your logic if notepad has a bug that's exploitable by opening a text file, it's NOT a windows issue.
That's logical given it's part of the standard system. It's unfair to any vendor to lump vulnerabilities in all software onto the OS. It's also unfair to the users, giving too rosy a picture, to include only the kernel and core libraries.
The middle ground is better. It has IE exploits counting against Windows, Safari exploits against OS X. QuickTime should count against OS X, since it is the OS's player, but not against Windows, but WMP exploits should count there.