Oftentimes today, free apps aren't actually free - when you take into account the massive number which include potential micro-transactions.
And this is a major issue for transparency for customers, something the European Commission has pointed out to Apple for their iTunes store. So they've opted to change the button you use to download an app with no initialpurchase price. It used to read 'FREE' now instead says 'GET'.
It may seem like a fairly small change but its significant in that it merely suggests you can now possess the app, but that there may be charges later for additional content.
The change has only rolled out in Europe and follows on from pressure on Google to change their listings - they no longer have tabs featuring 'Top Free Games' etc but rather just 'Top Games'. These changes are all tied into making it easier for consumers to realise the cost of an app up front, with the Commission also tasking Google and Apple with making sure they don't direct their calls for in-app purchases at children.
As always, make sure to check if an app is really free before you download it and ensure that a password is required before your account can be charged.