It's not exactly clear why Apple decided to change the wording of its buttons; however, it likely has to do with the controversy over in-app purchases.
Back in July, The European Commission issued a statement criticizing Apple for taking 'no concrete and immediate solutions' to address concerns with in-app purchases, in particular inadvertent purchases by children.
In December 2013, Apple, Google and the Interactive Software Federation of Europe where asked that:
● Games advertised as "free" should not mislead consumers about the true costs involved;
● Games should not contain direct exhortation to children to buy items in a game or to persuade an adult to buy items for them;
● Consumers should be adequately informed about the payment arrangements for purchases and should not be debited through default settings without consumers’ explicit consent;
● Traders should provide an email address so that consumers can contact them in case of queries or complaints.
Notably, Google agreed to make a number of changes, one of which was not using the word "free" at all when games contain in-app purchases. It's likely that Apple's change is a similar step to address EU concerns.
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