Seems fair to me. Why would anyone NEED to replace their battery twice in a single year anyway? They're already replacing the battery at a discounted price even if it passes diagnostic.
People could abuse it if they come replacing the battery every 6 months just because they're paranoid that their device has slowed down.
Hmmm, I wonder why people are paranoid? Apple's actions maybe? When a company loses trust from their customers, they need to earn it back. That has a cost. Dealing with a few paranoid people the right way might be the price to get the trust back.