Earlier quoted context omitted.
They could try. Apple would no doubt challenge both the NSL and the associated nondisclosure order (if one were issued with the NSL) in the courts, though. NSLs aren't magic.
Who's to say they haven't already tried actually? From what I understand, an NSL could be presented and it's validity argued but it would never be presented in an open court.
Both the validity of the NSL and the application of a nondisclosure ("gag") order to an NSL (not all NSLs are inherently gag-ordered) are reviewable by court, and gag orders have been struck down by courts.
So its not at all the case that an NSL would never be discussed in open court. Still, its impossible to say what NSLs have been issued with gag orders that haven't been struck down, since those particular orders would not be publicly disclosed.