I guess Lavabit figured that giving access to the data of one user in the requested way could compromise other users' privacy/security so that would be equal to warrantless wiretapping. Otherwise, what was the problem with following that?
Lavabit even admitted, they already assisted investigators in the past for investigation on a specific user. Apparently that request was legal in the view of the Lavabit owner, so there should be something different here.
It sounds like he was incurring some significant fines as well, so he was left with the choice of undermining his entire business model, or closing down.
I was talking about that unclear first request above.