I don't really understand why anyone who claims to be a proponent of a free market economy has an issue with a private corporation deciding what type of content they want to broadcast. JR sold his catalog of intellectual property (podcast episodes) of his own free will and was paid very well for it: about $100M from what I can gather. What Spotify chooses to do with it is completely up to them, even if they bought it…
But : the negotiation would have run very very different if they would have said :we buy it, but will never use or publish it!
He might have still sold (for a higher price) or rejected the offer.
You can't change the "purpose" of a contract after it was made.
In german, we have a law, yes a law, basically saying : in treu und glauben (in good faith / in trust and believe) it basically prevents contractors to write contracts which fuck over signers.
It is super rarely used for misuse of contract law / information of the signer, but sometimes it is. This is probably one of those cases.
In good faith, that Spotify will publish his work, he signed the contract.