The major difference is that for wifi, you need to setup your connection to whatever wifi network is provided. With your cell data service, that happens automatically for you. You don't need to go in and explicitly join some guest network and sign their ToS.

Maybe if there was some "cell->wifi" standard that could happen, but I'm somewhat doubtful about the benefits there. You'd also have to be super careful, for the same reason you wouldn't really want to join a random open network.