It has been tried in the past and may happen again sometime soon, but there are potential problems with non-subscriber tournaments. For example, when someone subscribes they are making commitment to the site with the idea that they're goign to be using it for quite a while. While some non-subscribers will have the same commitment many won't, meaning that the tourney falls apart as by the time you've got to the 3rd round (which can take over a year) most of the players don't come to the site any more.