I assume it has to do with the official branding of both Google Play and the Apple App Store. The official button images that they've approved use the verbs 'ge

I assume it has to do with the official branding of both Google Play and the Apple App Store. The official button images that they've approved use the verbs 'get' and 'play' respectively, and I believe it is probably against their TOS to use non-approved graphics/text/branding for download buttons.