NSL is a statutory authority document issued directly by the executive without judicial involvement. It is not a legal proceeding nor a warrant, nor is it even on court letterhead. There are no statutory penalties for noncompliance set out in the law defining NSLs, but it has provisions to
request a court order to enforce if the recipient does not comply. That requires filing a federal case, bringing the intelligence operation to the attention of the judiciary, and probable argument with an opportunity for the target to argue. This is where you hear about folks like EFF defending an NSL, since replying to an NSL usually does nothing.
After a court issues an order, contempt of court is a possibility. Just clarifying that the route to contempt is not quick. It’s also largely untested. Writing an NSL is two pages in a Microsoft Word template, while arguing a federal case to get your way is a much bigger prospect; if the investigation is small enough, or they’re not totally legal in how they got intelligence, etc., etc., they might not wish to argue and calling the bluff might be smart.
The gagging facility of NSLs actually has a non-coercive purpose: as designed, an NSL basically invites an unknown third party into a sensitive intelligence or counterintelligence operation. Tipping off the target or anyone else could lead to a collapse of the investigation, burning other sources that were used before you got your NSL, diplomatic repercussions, and so on. That’s the thinking that went into it, and it’s actually understandable. Two problems are that (a) the gag is indefinite, with no circling back once the operation concludes and (b) NSL is horrifically abused for stuff it shouldn’t be, since FBI realized the gagging lets them mostly get away with it.
Source: Have held more than one and read the citations.