We would like to keep the game free as a download for a couple of reasons. I know some of us devs have sig others that have an iPad, and they maintain their own iTunes account. One thing we didn't want to do was force people to pay for another download on a different device.
Part of the issue that comes in to play is there is no way for us to tell if you bought a copy from the store, or pirated a copy. To the servers we just see the game connect. So we would have to assume if we see a new device you bought a new account. That would mean that any new device you install it on (let's say you have 1 iTunes account with 5 devices) we would have to assume you bought another month. There is no way of us knowing if this is the same iTunes account or not.
It just adds a layer of complexity that isn't required. Sure we fully expect people to complain and give us 1 star saying there is a subscription even when the first 2 lines of the description clearly states that. All we can hope is that people that like the game post a review to counter the 1 star free complainers.
In the long run we plan on doing something to let people try and play the game, but it will be in a limited fashion and they will not have interaction with the paying players.