Copyright Issues ...

In a recent issue of The New Republic, I read the following:

A scientist's plea for Christian environmentalism.

Apocalypse Now

by Edward O. Wilson
Post date 08.28.06 | Issue date 09.04.06

Because of copyright issues, this article is not available online.
Subscribers can download it in PDF format here. ...

What are these copyright issues? How do they prevent the article from being posted in HTML while permitting it to be posted in PDF? What's going on here?