According to a Google Play support document -- and confirmed when we browse to some magazines -- Google is now offering a "Free for print subscribers" option for certain magazines in the Play Store. When browsing many popular magazine subscriptions in the Play Store that have a print component, tapping on "subscribe" offers you three options. On top of the regular monthly or yearly subscriptions, you now have the option to get the digital version for free if you're already a print customer.
Google has likely signed some kind of content deals to make this happen, but however it happened it's a good thing. Many different publishers have offered this feature for their own websites and magazines, but its nice to see Google offering the same option with a digital subscription through the app as well.