Microsoft created the requirement to have secure boot with Microsoft keys, knowing very well that the incentives created by that requirement would lead to companies producing motherboards that can only use that and nothing else. Microsoft would not be able to do things that create these incentives if they didn't have a monopoly.
Just because the OEMs can choose not to lock down the hardware doesn't mean that it's the OEMs' fault rather than Microsoft's; the incentives were created by Microsoft.
Remember back when Microsoft were simply creating volume license agreements that made it expensive for companies to ship computers with Linux? They could do it--it wasn't prohibited by contract, it was just more expensive. All Microsoft did was change the financial incentives. But that's enough that it should be considered Microsoft's fault.