They didn't necessarily do it because they wanted to "stop teh NSA." A lot of people who wanted to "stop teh NSA" started using TrueCrypt, and so they assumed that their goals lined up with TrueCrypt's. But maybe they didn't.

Maybe the developer using TrueCrypt was perfectly happy with "defend against anyone short of the NSA, especially since the NSA would need to expose their ability to break into this in order to do anything bad to me." There are millions of people who legitimately share that threat model.

We can parse out each comment in the source code like lawyers fighting about a comma before SCOTUS or biblical scholars debating on the definition of a word in Hebrew. We will never know. But there is a really big possibility that the developer(s) consider BitLocker acceptable, even if it's closed-source by Microsoft.

EDIT replaced an instance of "BitLocker" with "TrueCrypt" in second paragraph, whooops!