That's true in many important Constitutional areas, in many cases because parts of the Constitution are generally read in a way which incorporates that view (its particularly true, e.g., of things like 8th amendment "cruel and unusual" standards, 4th amendment reasonableness, 9th amendment nonenumerated rights, and 5th and 14th amendment due process.)

That is, the ways in which the Supreme Court "acts like a classic common law court" are because certain written sources of law like the Constitution or particular statutes are understood to require the court to do that to apply the law as intended, because the text itself invokes considerations that require consideration of the social context to apply.

xkcd has an interesting graph of this: https://xkcd.com/1431/

I hope, for the sake of American democracy, that this just isn't true.

And I mean that literally. I started typing about five different responses, each beginning differently. None of them seemed to work, and besides I would've probably ended up typing too much anyway.

Instead I refer you to:

1. https://en.wikipedia.org/wiki/Separation_of_powers

2. https://en.wikipedia.org/wiki/Legitimacy_(political) and http://plato.stanford.edu/entries/legitimacy/