it was to commemorate the release of pocket legends for the Xperia Play phone. same way they released the first droid antenna (green) to commemorate the release of PL on android devices.

if you are asking why were xperia phone users only granted this vanity, that is false. you had to find the devs in game via announcements on facebook and twitter during this event, and wave at them as they ran around through townes (they didnt make it too easy lol)