That's because Weather Underground's data is significantly more expensive than Dark Sky's. It's annoying, I agree, but I either had to charge extra for it for Apple Watch users or not offer it as an option at all.
$10 per user per year? It should be clear what you get as I had no idea there were 2 tiers of Premium Memberships so the one I purchased was a waste of money as I doubt I’d pay $10 per year.