Reddit's Data API TOS has always allowed it the right to start charging for access. That it chose not to do so until recently was its prerogative. That it chooses to do so now, is also it's prerogative.

This is not a unfair taking, since Reddit isn't taking anything from Christian, they are simply no longer freely providing something.

This is not an issue of estoppel, since Reddit never promised to make their API free forever. And Reddit gave him due notice, as required by their TOS, of changes that would take effect...several months after notice was given of the changes...

This is not tortious inteference, since Apollo could have continued to provide Reddit services to their customers, though this might have required Christian to change his business model.

This is not slander, since on the call Christian clearly suggests to Reddit to give him $10 million and he'll go away and not make a fuss about things.

It's irrelevant that they have a "prior relationship" since that means nothing in this context, since Christian did not have a binding contractual relationship that entitled Christian to perpetual free access to the Reddit Data API.