They could be planning to use some sort of micro-transactions...

They could be planning to use some sort of micro-transactions model, that is quite common in casual games these days. Build up the player base while it's free supported by advertising, then once players are invested in the game release extra functions that you need to pay a small amount to get access to.

It's a slightly different beast, but its worth looking at the recent changes to Dungeons and Dragons Online. The game was struggling while under the standard MMO subscription model, so they completely changed the business model so the basic game was completely free, but players could pay small amounts to buy extra content. The game now has over double the amount of subscribers and is making 500% more revenue than before.

http://www.gamesindustry.biz/articles/dungeons-and-dragons-online-revenues-up-500-percent