Forbes has a bit more information on the closing of Lavabit. They provide additional clues that Ladar Levison of Lavabit was indeed asked to monitor users’ logins in order to capture their keys.
Among other things, we learn that Levison has complied with “two dozen” individual warrants in the 10 years that he has run Lavabit. He says explicitly that the reason he shut down Lavabit was not because of a request for a single user’s data but to protect all his users. That’s a bit ambiguous, I suppose, but a fair interpretation is that the government wanted him to do something that could potentially affect more than a single targeted user.
I’ll leave you with this quote from Levison explaining why he is stepping away from email:
If you knew what I know about email, you might not use it either.