I'd be amazed as well, they're far better off using it to promote themselves and try to bait you to try/look at other things. Paywalling it would just remove most of those daily users they just bought. Maybe when it dies off, then they will do that, but quite likely most people will use one of the many clones, especially the apps. The fad will die off really quick I'm thinking. To me it doesn't have the lasting power of things like sudoku.