This is one of the things that I've always found to be a little odd --- it's one of the simplest ever DRM schemes. My theory is that Knuth put it there just to satisfy his publisher's lawyers, since otherwise he wouldn't let the source be copied freely, much less make it public. Being who he is, he could've also made it much harder to bypass. Rather clever of him, and it somewhat fits with his style.

Of course I could be wrong and he truly has quite conservative views on IP...