Amazon has been experimenting with its Amazon Fresh pricing for a while now. Initially costing $99 a year plus delivery fees, and then $299 a year including delivery fees, the service has been charged at $14.99 a month since 2016. Bundling it in with Prime represents pretty good value, then.
It's likely that Amazon will lose out on some subscription revenue, but the move could encourage more people to actually use the service, which is where Amazon will recoup any losses. The company hasn't revealed any details on the numbers involved, but does say that grocery delivery is "one of the fastest growing businesses at Amazon." Indeed, it's facing stiff competition from other companies such as Walmart, Postmates and Doordash -- and in the UK grocery stores already offer this kind of service as standard -- so it makes sense that Amazon would be pushing extra hard in this area.
Update 3:21PM ET: This post has been update to clarify that free deliveries from both Amazon Fresh and Whole Foods are available for Prime members.