The problem with that logic is that Microsoft doesn't see its customers as purchasing its product, rather it sees a contractual agreement set-up similar to renting an apartment where Microsoft is the building owner and you're the tenant. In that case, providing everything inhouse looks to be preferrred to tenants. Their logic is that since you wouldn't want to cart 7 loads of laundry down 5 stories and 3 blocks away, your landlord provides washers and dryers, and since you're using a computer with Microsoft Windows, why would you want to go buy separate media playing software when Microsoft can provide it for you effortlessly.
The downside to this setup is that if your roof leaks, you can't just go fix it yourself, the owner must have it repaired. So if Windows Media Player is causing your machine to hang and you have items in Startup that launch WMP, you can't remove WMP to regain use of your system.
I <really> hope Microsoft fixes their logic soon, for their sake. Most PC owners see themselves as owners and not renters so having a lender/leaser relationship ranges between inconvenient and infuriating.