We know you just want to be able to play the game, and we really want to make that as easy as possible.
The reason the updater needs a key is to handle the case where a key gets publicized. While we're happy to provide free updates to our players, we can't do that if our server costs become unmanageable. Having keys allows us to keep the costs associated with distributing the game to within a healthy limit.
You don't need a key to install the game on multiple computers, and you can even use the same key to update multiple devices.
If you would like to disable the updater, the first dialog box that pops up has a check box titled "Disable Updates". Check that and you should never be bothered by it again.
We hope this strikes a healthy balance between ease of use, and our ability to provide free updates for players. If you have feedback or questions about our updater, please let us know in the forums!