Why not let fans make money even if most of it went to CBS? Because doing so opens a whole can of worms in terms of contracts, fees, and accounting. Heck, just the time it takes to draw up the contracts can be expensive. I've sat in on contract negotiations between Amazon and a software company for a SINGLE work for hire software project, and it took months. The amount of money CBS could make from such a deal would likely never amount to their legal or the fees they'd incur to do an audit. Just. Not. Worth. It. (I suppose they could draft a boilerplate take-it-or-leave-it agreement, but, again, the legal costs incurred if a breach occurred still likely wouldn't make it a profitable venture.)
You are mandated to pay SAG actors for most productions, but the Short Film Agreement allows NO payment for certain film categories and deferred payment for others (section 5 here).
Thanks for the explanation. A take-it-or-leave-it kind of thing was what I was thinking of.