Why would fusion power necessarily be limitless or cheap? Even if a friendly alien starship chose to beam us down a production-ready fusion reactor as a welcome

Why would fusion power necessarily be limitless or cheap? Even if a friendly alien starship chose to beam us down a production-ready fusion reactor as a welcome-to-the-neighborhood gift, it'd be kind of a white elephant: the steam-turbine generator we'd have to build in order to capture the reactor's power output as electricity would cost more than we'd spend on equivalent renewables.