They wouldn't necessarily need to be for free. Porting it to another platform opens up a new market. There are probably a lot of people that have an Android phone that haven't bought Minecraft for iPhone or PC.
> Minecraft is a sandbox building indie video game...It was released on May 17, 2009, with a Beta on December 20, 2010.
http://en.wikipedia.org/wiki/Minecraft
Years, actually.