[gnopgnip]

Where in US statutory law or case law is there an implicit right to revoke a license unilaterally?

There is plenty of case law that says the opposite. For instance Cohen v. Paramount Picture shows that copyright licenses are not unilaterally revocable even if the original license doesn't explicitly state it is irrevocable.

A license is an enforceable contract. Revocation requires the consent of both parties, if no exception is provided for originally. Eisenberg, The Revocation of Offers covers many of the nuances of when a contract or even an offer is no longer revocable.