Media outlets lately have emphasized the challenge of enticing healthy young adults to sign up for ObamaCare, "exactly the type of person insurance plans, states and the federal government are counting on to make health reform work," as the L.A. Times put it. These pieces are useful as far as they go, but miss a key point that Supreme Court Justice Samuel Alito managed to convey in many fewer words during last year's Supreme Court argument on ObamaCare.

Mr. Alito pointed out that young, healthy adults today spend...