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 for iPhone doesn't exist, so that's out. Minecraft for PC has existed for months. I could see it drawing in new people with the promotion a new port gives, but I doubt it would be enough to cover development costs considering how many people have already bought Minecraft PC.
