I know, people will say, this hardly makes a difference, right? But all the steps hardly make a difference. OSX Mountain Lion defaulting to only allow apps signed with a certificate from Apple? Why, the user can just disable that ... not including Flash or Java? Sure, just install them! It all hardly makes a difference. But all these steps put together - they make a difference!
Slowly and surely we're being led like lambs to the slaughter to a world where everything we say or do with our technology will be under the control of giant tech companies that ultimately care only about their own profit. This is just one more little step to ensure that developers preferentially make apps for Microsoft's walled garden rather than free apps that run anywhere and might compete with their interests.
Edit: correction about Mac App store vs signed apps