Your Windows Defender argument seems like apples and oranges. It doesn't fully prevent adware infection anyway, but regardless, the discussion is about whether a hardware maker (or platform owner if you're going to try to extend this to OSes) should be allowed to prevent 3rd-party software selected by the user from running on their platform.
Defender doesn't do this: it's an optional security application by MS that's included with Windows. Users are free to disable it if they wish. Nothing is preventing Windows users from running whatever software on top of Windows that they like. Defender will prevent some malware from running, and many users like this for obvious reasons, and the fact that it's included for free unlike competing anti-malware software, but it's not so baked into Windows that you can't turn it off. An antitrust argument could possibly be made, along the lines of Windows including IE and putting competing software out of business, but that's a different issue than what we're discussing.
So you're right about this being really an OS vendor issue, but Microsoft doesn't force anyone to use Defender, and doesn't prevent anyone from using competing products. Google also allows using competing app stores (or even side-loading .apk files), though only a tiny fraction of users take advantage of this. Apple is really the problem here because it doesn't allow these things at all.
Defender doesn't do this: it's an optional security application by MS that's included with Windows. Users are free to disable it if they wish. Nothing is preventing Windows users from running whatever software on top of Windows that they like. Defender will prevent some malware from running, and many users like this for obvious reasons, and the fact that it's included for free unlike competing anti-malware software, but it's not so baked into Windows that you can't turn it off. An antitrust argument could possibly be made, along the lines of Windows including IE and putting competing software out of business, but that's a different issue than what we're discussing.
So you're right about this being really an OS vendor issue, but Microsoft doesn't force anyone to use Defender, and doesn't prevent anyone from using competing products. Google also allows using competing app stores (or even side-loading .apk files), though only a tiny fraction of users take advantage of this. Apple is really the problem here because it doesn't allow these things at all.