The FAQ is saying something entirely different:
Do I have to be a SKY customer to access this service?
No you don't. SKY Sport Now is a sport only offering and can be purchased independently of a SKY subscription.
I don't see why they would care, all in all, it's extra money in their pocket.
EDIT: They do reference SKYGO for existing sport subscribers, but again, extra money in pocket, I don't see why they'd care.
Its in the Full FAQ
If I am a SKY SPORT subscriber do I get access to the SKY Sport Now app?
No you will not have access to the SKY Sport Now app. Remember if you're a SKY Sport subscriber you can access SKY Sport 1‐4, ESPN and pop‐up channels on your set‐top box and with SKY GO available on PC/Mac and selected tablets and mobile devices.
To me, that is just stating you don't get access to it bundled with your subscription, not the fact that they won't let you pay for it?
Its saying you are not eligible. Try asking for it with the Sky name, address and email