He had to give up other peoples' privacy, as well.
The alternative (that the FBI demanded until he started playing childish games) was actually only handing over Snowden's mails.
Which, to be clear, would still have been a disaster for Lavabit! It's not my suggestion that he should have gone along with the FBI and then continued to run the service.
But had he complied, as he was legally obligated to do, with the FBI, he could have immediately and gracefully shuttered the service without having given up the TLS keys that unlocked all communications to the site.