There are methods which exist where you can 'spoof' your machine's hardware to appear as though it's a genuine Windows OEM, using a 'wrapper' boot loader.
A secure UEFI prevents that method, and a hack needs to be found in the Windows boot-up chain of trust (Much like how you would hack an XBox 360, which is very hard.)
Since the most recent UEFI standards permit the option to turn off secure boot on intel machines, some might consider this to all be moot anyway, since people will be running unsigned code whether this 'pre-loader' exists or not. However, the UEFI standard merely *permits* an insecure mode. It doesn't enforce it. The hardware manufacturers are allowed to require secure mode, if they so choose, and still remain complaint with UEFI. Also, the ARM UEFI standard completely forbids an insecure mode, and Linux runs there too, so the Linux Foundation should by all rights be planning on getting a similar loader signed on ARM as well.
Long story short, if Microsoft chooses to sign this, it would be a win for both Linux, and for people who like to get unauthorized copies of Windows. Perhaps this means they will refuse to sign it, and invent some excuse that it will compromise their user's security or something equally absurd.