I just wanted to step in and clear any ambiguity here.
Andre is correct.
Google almost certainly does this on the back-end. It appears that they show this button for results leading to the actual Apple App Store and they probably pull in a little bit of meta data directly from Apple. So you will not be able to control this.
And everyone else on the thread,
I just double-checked that Google has definitely implemented this feature: you can open the developer tools in Chrome, change the user agent to an iPhone, and you will indeed see Google's "Free" buttons.
Apple most certainly does not modify or inject random HTML or JS into the DOM in mobile safari. That would be ridiculous. It would be unsafe, inefficient, insecure and against web standards. Developers would be in an uproar. In terms of WebKit's relationship with Apple, please read this overview on what WebKit actually is (it's quite complex): http://paulirish.com/2013/webkit-for-developers/