Code trumps law on this issue anyway. NPR has the ability in principle to check referrer links, or to use randomly generated transient URLs for stories, or to use other technological measures to prevent deep linking. Given this technological reality, it is logical for the law to be consistent with it.

Granted, NPR has not actually gone to the trouble to do this (yet). But why is it so awful for them to impose rules which they can enforce if necessary?

It's like the difference between a "No Trespassing" sign and a brick wall topped with barbed wire. If you have the legal right to put up the wall (as NPR has the legal right to use technological measures to prevent deep linking) then putting up the sign (analogous to asking people to get permission before deep linking) should be acceptable.