Not even the PR implications, but just the security/technical. How does an OS vendor block access to a website? Deny it in the hosts file? Add a default firewall rule? Code a special case in IE?
Any such incident would cause irrevocable damage with any government/enterprise and push them to abandon the platform entirely. As you say, it's just absurd.
Like a computer where you can only buy apps from a single site owned by the maker and only connect to the net through a single service provider - no consumer would ever go for that.
Absurd? Ludicrous? Okay, maybe it's a bit of a stretch, but if you look at Microsoft's long track record of harming consumers, I wouldn't bet the farm against it.
Let me see... how about not supporting HTML5? That would pretty much kill all of the new Google products for 70% of the PC market... And it's not immediately grounds for an anti-trust lawsuit.
Any such incident would cause irrevocable damage with any government/enterprise and push them to abandon the platform entirely. As you say, it's just absurd.