Your intuition about fairness matches a principle recognized by courts. The term for the applicable legal doctrine here is "estoppel". There's a strong argume

Your intuition about fairness matches a principle recognized by courts. The term for the applicable legal doctrine here is "estoppel". There's a strong argument to be made that if Instagram didn't make its (fairly unorthodox) position on embeds clear beforehand, then Newsweek's original use was reasonable and non-infringing (but not something that it can rely on indefinitely, esp. after Newsweek was notified that it did not have permission). There's another argument to be made, though, that the absence of an explicit prohibition is not necessary, due to copyright being an automatic negative right—that the burden is on Newsweek to obtain a clear license, not on anyone else to do any work explaining to Newsweek that it can't use it. This is a strong, argument, too, but probably weaker than the argument from estoppel.