Are they shutting down preemptively because shutting down a service that has received an NSL is illegal? I'll be watching what happens to Lavabit to find out.
If they get rid of the service and the data pre-emptively, they can safely destroy the data. Once they've recieved an NSL, they have to comply with the order and provide the data (or at least the metadata) or be in contempt of court.
NSLs come with gag orders, and it's possible that the government would view shutting down a service entirely as a violation of the duty not to disclose. It gets fuzzy here, because we're speculating on how a secret court will rule on a body of secret law while working hand in glove with the NSA.
IANAL, but a reasonable guess tells me that while shutting down the service wouldn't be illegal, you would still be on the hook for the requested data. You don't pony up, the Feds would, at a minimum, charge you with obstruction of justice and consequences would follow.
