They can't (feels weird to say "can't" and "mandate" for an open source project)
Open source isn't relevant here because that mandate is for any vendor wanting to distribute Google's proprietary apps (Market, Maps, Gmail, Google account sync, Face unlock, etc.) with their device. Every carrier and hardware vendor except Amazon falls into that category and is going to have to do whatever Google says to continue licensing those apps.
Yes, they can continue making their own themes, but unless app developers specifically choose to use it with DeviceDefault, those apps will use the unmodified Halo theme which must now look the same on every device.
Open source isn't relevant here because that mandate is for any vendor wanting to distribute Google's proprietary apps (Market, Maps, Gmail, Google account sync, Face unlock, etc.) with their device. Every carrier and hardware vendor except Amazon falls into that category and is going to have to do whatever Google says to continue licensing those apps.
Yes, they can continue making their own themes, but unless app developers specifically choose to use it with DeviceDefault, those apps will use the unmodified Halo theme which must now look the same on every device.