"I didn't refuse to transport your goods, I just said it would cost a billion dollars per pound to do it and you couldn't afford it" is not the gotcha that you think it is. The law is technical but it's enforced by humans.
It's straightforward: Apollo and Reddit have a longstanding business relationship, via these APIs that Reddit has provided for a long time at zero cost. Reddit generally no longer wants third parties to use the API, so they are increasing the price to a level that they know will cause everyone to balk (other third-party clients are closing up too) so that they can direct that traffic to their own native client and first-party sites, while knowing that Apollo has these long-standing business relationships of their own that are built on this relationship with Reddit.
In short, reddit is deliberately taking action to sabotage and cause economic harm to a business partner by changing aspects of the relationship that make it impossible for the partner to fulfill their contracts to third parties, so that Reddit can direct that business to themselves instead.
That is an improper taking under tortious interference, and the rest of the tests (intent actual economic loss - not just refunds but future income, etc) are trivially satisfied here.
I know people are libertarians here but the right to swing your fist ends at someone else's face, and legally speaking if you take actions that you know will result in a business partner being forced to sustain economic losses due to your improper breaking of your business relationship with them, you are generally liable for that damage you cause to the partner. That is the basic concept of tortious interference, you're paying for the damage you caused to your business partner. Swing your fist and hit someone's face and you get to pay for the surgery.
(IANAL and Reddit's lawyers would obviously say their conduct is proper, but, generally this is the type of situation where people can unexpectedly get themselves into legitimate legal trouble based on actions they think are perfectly legitimate. And generally they may have been legitimate if you didn't have this prior relationship, that changes things! It's different to not build an API at all, vs having the API be free and have third parties start selling clients and then to stop doing the API.)
(As a sibling comment notes, estoppel is another - if you promise something to someone, even a verbal promise, and they take a financially detrimental action on the expectation that you will follow through on your side of the promise and you don't, then you are generally liable for the financial harm you have caused them too. Libertarianism doesn't mean you can wiggle out of contracts, even verbal ones.)
