Much of the 'idle' capacity of satellites is allocated on what is called a "pre-emptive basis", satellites are known to fail, rarely but it does happen. So most satellite companies keep a little capacity in reserve so that when one does fail others can pick up the slack. Most of this "spare" capacity isn't idle all of the time, in fact the TV industry uses it extensively for news gathering, relaying sports events, big corporates use them for product launches and internal communication, there are many "occasional use" business cases.
It might be possible to rent this idle capacity on a parasitic basis, you could keep the transponders lit when they are idle and taken them down before they are needed. But your receivers would have to work on the basis that they expected the connection to fail and sometimes be totally unavailable. Plus I would be nervous about allowing these terminals to transmit back because although they should only transmit when the spectrum is available they could possibly contribute to unintentional jamming of commercial traffic. So it would be easy to deliver high bandwidth data on a unidirectional basis using just spare hardware and spare capacity, getting bi-directional access is way more challenging and would require significant investment.
(Bio: I spent enough years doing satellite comms into weird and wonderful parts of the world for a two large organisations).