It’s been like that for years.
To get the $14.99/month rate, a sub is required. To do that, you need to provide a valid credit/debit card that they can bill automatically each month. That cannot be done through Blizzard Balance.
And using Blizzard Balance, you can only purchase 60-day blocks of time.
[Editting to add…]
Much thanks to Mirasol, as always for this handy dandy chart
