In that specific case, it's because Nespresso's patent on the capsule design ran out and they can't stop other people from selling compatible capsules anymore.

In that specific case, it's because Nespresso's patent on the capsule design ran out and they can't stop other people from selling compatible capsules anymore. While they still had the patent, they went after third-party capsules pretty aggressively. There is no "fair use" clause for patents or trademarks the way there is for copyright, but patents expire much faster. (Trademarks do, too, although they can be renewed indefinitely -- but you have to do it affirmatively every 10 years in the US.) With the necessary "not a lawyer" disclaimer in place: in the case of Reddit's APIs (or anyone else's), AFAIK the issue wouldn't a trademark or patent issue, per se -- an API can't be patented (or trademarked), and it's debatable whether they can be copyrighted. (A case between Oracle and Google involving Java's APIs made it all the way to the Supreme Court, which kind of punted on making a definitive legal decision as to the copyrightability of APIs.) But, making unauthorized use of them could almost certainly be classified as a breach of contract, e.g., a violation of the end-user license.