More often, it seems to me, "constitutional principles" are those that the speaker thinks ought to be incorporated into the Constitution, even though they aren't.
At Larry Solum’s Legal Theory Lexicon: Formalism and Instrumentalism. From the description of formalism: The core idea of formalism is that the law (constitutions, statutes, regulations, rules of procedure, and precedent) provide rules and that these rules can, do, and should provide a public standard for what is lawful (or not). That is, the core of […]