From the comment box: Reen suggests that without the appeal to intention, the Constitution (or, I guess, any other document) is just "accordionlike." I'll admit I actually like--or at least don't mind--the image of the Constitution as accordion, each article and amendment opening out in possibly infinite ways. But that's me being a naughty lit-crit type.
I'm still a little puzzled, though, by the assumption that both Reen and Tim Peterson make: that it is only an appeal to authorial intention that can constrain meaning. The idea is that without an idea of intention, a text could just mean anything.
I guess that just strikes me as strange. Thanks to the lingering legacy of New Criticism, "intention" is about as dirty a word in academic literary circles as you can get; yet the rejection of intention doesn't just allow a critic to say whatever he or she wants. Saying that we can't know precisely what Shakespeare really intended by writing, say, Romeo and Juliet (or that we can't know with absolutely certainty that Shakespeare even existed) doesn't mean that we're then free to read R&J as an allegory of the development of the airplane.
Literary criticism has largely replaced intention with the more flexible idea of historical context, which does try to understand things like what a certain phrase or word might have signified to the author's peers. But the larger point is that this is only one of innumerable factors that go into a claim about what a text means; to interpret is to negotiate among these factors, not to adopt one of them as an absolute limit.
And interpretation itself has a history--as I think our legal system recognizes. In interpreting a law, especially one more than a few decades old, I'm guessing that very few judges are going to look directly to the intent of the legislators of the original bill. Instead, they likely rely on precedent--the history of how that law has been used and interpreted by other courts--because that is probably more useful in understanding the interface between the text of a law and reality than the idealizations of the person who conceived the law. It's also a notion that acknowledges the possibility of change and development, rather than a meaning fixed for all time--yet still constrained with the framework that the text of a law lays out.