I think your loose definition of “principle” does not meet many others’: it’s definitely intended to be stricter than a heuristic.
Of course even though you can have relaxed principles, e.g. “I usually don’t lie” vs. “I don’t lie,” I think the point here is that if you believe or preach a particular “principle,” but then adjust it for yourself when it’s convenient (for example a “just this once” scenario), it was never a principle to begin with.
Actually, observationally I believe my definition of principle to be far more accurate. People self-report principles as being far more immutable than they behave like.
I have not yet met someone who rigidly adheres to theirs. Perhaps if I had met the Dalai Lama but nearly everyone else will find that the moral space is so fraught that they cannot adequately compress it into immutable laws.
Usually, should one demand such adherence one would find one met with accusations of “looking at the world in black and white” and “lacking nuance”.
Of course I don’t really intend to change your mind, but I do intend to transmit what I have observed.
Of course even though you can have relaxed principles, e.g. “I usually don’t lie” vs. “I don’t lie,” I think the point here is that if you believe or preach a particular “principle,” but then adjust it for yourself when it’s convenient (for example a “just this once” scenario), it was never a principle to begin with.