This being the pared-down NYT, the error managed to make its way through the editorial process and into the print edition this morning -- reminding readers of the paper's declining resources in copy-editing a newspaper that once prided itself of getting everything right.
In a news quiz on grammar published in November of 2008, this is what now-standards editor Philip B. Corbett had to say about the confusion between the words "principle" and "principal:
Here's a screen grab of the offending passage.
In case you're wondering, here's the rule -- as provided by the NYT-owned website, About.com:
As a noun, principal commonly means "administrator" or "sum of money." As an adjective, principal means "most important." The noun principle means "basic truth" or "rule."