APIs should not be revenue generators! I don't mind companies charging for an API, either to cover the costs of service or to encourage efficient behavior. But Twitter and now Reddit seem more like they are rent seeking with their APIs and it's just not going to work out well. Particularly galling since they're effectively charging to access all this content that users generated for free.
The other explanation is these charges are intended to kill third party uses of the API. I'm pretty sure that's what is mostly motivating Twitter (down to the weed joke price).