"People buy Tetris, and they can get that anywhere."
That's true. Though back in 1989 it wasn't quite as easy to get a hold of which is probably why it did so well for the Gameboy. Plus back then there were no cell phones for people to casually game on the go. That's a good example though. The only major difference I can think of is that Tetris is popular among serious games while Solitare and Pop Cap games and stuff like that isn't so much.
Does anyone know how well Bookworm sold on the GBA? That's a perfect example of free game being made available for sale.
I'm also curious what Teris Worlds sales for the GBA are like. That's really the first portable Tetris game released in an era where everyone has a cell phone and the first major portable Tetris game that isn't made by Nintendo. I imagine "Nintendo's Tetris" sells far better than regular Tetris games typically do.