Oh, so you're one of those "living document" liberals for whom words have no meaning. "Public purpose" can be... well, anything. You're the one who's being absurd, Bunky.
What that really means is that the decision and interpretation of the ambiguious term material to the case was decided in a manner opposed by the person using the critical and pejorative descriptions.
Law is necessarily unclear and ambiguous otherwise judges would have nothing to do and the law would be the same a the French system where, if it's not in the statute books, it doesn't exist. That is not consistent with the common law tradition of American law. The very first statute in the U.S. Code and every state, except Louisana, recognizes the common law of England as it existed on July 4, 1776 and incorporates that common law into our law.
And, although the Erie Doctrine of federal common law does not any longer exist, there is, nonetheless, a vast body of common law cited in our decisions and court decisions.