When you purchase the game, you pay for it as it is right now. Future updates are an added bonus.
Sure. So far the game has been updated regularly, but they explicitly stated that they are not guaranteeing any updates.
During alpha the rules were: "if they release updates, you get them for free. But you agree that when you pay, you pay for the game as-is and you don't have any right to expect updates".
Meaning: "Maybe I lose interest in doing this and just stop updating it. You have no right to be mad about this. If I update it though, you will get that update for free"
Convenience is the differentiator (itunes v.s. thepiratebay) and many consumers are willing to pay a premium for it.
Regardless, if you just accept that a good portion of your market won't pay for your product and work around it you can still have a very successful business. Then focusing on techniques like convenience, reward systems, free t-shirts? etc. can increase your paying customers. Maybe suing them can too, but it's definitely not the only way.