That would explain it. However. That is stupid as hell. People sell OEM comps with copies of Nortan or Mcafee, think of all the pain.
You haven't thought this through. MS' own statistics show that there's a sizeable portion of the userbase who either never bother to install an AV package, or let the license lapse. Packaging their own solution into the default install solves those issues. If an OEM wants to put another package on there, they're free to do so, but the fact of the matter is that a vanilla Windows 8 installation is now much more secure than a similar Win 7 install.
The only people this will cause pain for are other AV vendors, as they now have to actually work harder to prove that their solutions are better than MSE.
1% of a million is still 10,000, and "sizeable" could mean anything from 5% to 80%.
I can also say, from personal experience (which is already worth far more than a random MS statistic that i'm sure very few people opt-in to anyway), that people do in fact take the BesyBuy sales guy at their word and get Mcafee or Nortan and do install it, let alone systems that come with it or, even worse, pre-installed. They also do let the licence lapse. See above about it being the leading cause of problems I end up fixing.
MS putting MSE in Win8 by default will not fix any of the above issues. Sales guys + general consumers not knowing better are the best to blame here.
It does, however, provide a solution for those who are smart enough to not listen to the sales guy, need good free AV, don't know enough to install an AV, or as something I can offer over their now expired Nortan that decided to nuke the system's preformance.