1. Fantaisie-Impromptu is public domain. That only happens when the IP rights expire or are forfeited. So this is a different situation.
2. The copyright protections on the IP is not to protect a trade secret, so it doesn't matter how many people know or are familiar with the game.
3. The Tetris Company was formed and the creator of the game started getting paid. They copyright protections is for the people that own the rights to the property and still actively defends and use it, to retain control so they can continue making profits. The still make tetris games. There is one even scheduled for the 3DS. So are they right in protecting their interest? Sure they are. I believe ANYONE should be entitled to the same protections whether it is an indiviual or a the company that forms thanks to an individuals work. The implication that because they become a successful business that they shouldn't be afforded the right to protect their IP has to be one of the silliest things I have read in a long time.
4. Have you seen the game people are mocking? It is like a carbon copy clone. I would understand if this was a scenario of them going after a title that only barely resembles tetris (like lumines) but no this game is a clone. Down to shape, design and function. Even the blocks are the same. How is a clone like this beneficial in anyway?
http://wmpoweruser.com/wp-content/uploads/2010/12/Tetrada11.png