The problem was that following it would be a violation of the privacy of Edward Snowden (well, of the user whom we presume to be Snowden) and of the trust which he had placed in Lavabit.
Violation of his privacy under warrant is what the law would expect in that case. Violating privacy without a warrant (and probable cause and etc.), that what is unconstitutional. Or you are saying that there can be no investigation at all?
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.
