You just answered your own question.
Sometimes, principled leadership means signing crap like this, to make sure it gets killed in the courts vs. hoping it never comes up again.
Let me tell you why President Dubya signed "this piece of dung." One reason and one reason only - SIGNING IT FOREVER REMOVES THE ISSUE FROM THE DEMS. The Dems gave up the issue in return for a bill. Although they have preferred having the issue for years, this year, they made the strategic blunder to trade that issue for a flawed bill.
What will now happen, which would NOT had Bush vetoed it, is that the courts will declare it unconstitutional. Net result - the DEMS will have lost BOTH the bill AND THE ISSUE.
That's why he signed it. He avoids the media whitewash, which was sure to have been massive (since they haven't gotten anything to stick to him in 9 months), and he defeats the Dems by taking the issue away from them and exiling it to the trash heap. Damn fine West Texas strategery, if'n ya ask me.
Any time you can surgically remove an issue from the Dems, it IS a victory.
Michael