That policy has been in place for 2.5 years, since the original AppStore terms and conditions. It wasn't enforced because before now, Apple didn't offer SDK support for subscriptions. People have had 2.5 years to prepare.
A law is only as good as it is enforced. There's a huge amount of laws out there that would be very controversial if they started getting enforced. For example, many states still have crazy/puritanical sex laws on the books.