That doesn't seem like the right interpretation. They asked for this because Lavabit was unable, or possibly unwilling, to turn over Snowden's records, and impersonating Lavabit the next time he tried to check his email would have been the only way to get that info.
So yes, if your shopping list can't be recovered without your private SSL keys, they will take those when they have a warrant for your shopping list.
According to the New Yorker piece on Lavabit yesterday (http://www.newyorker.com/online/blogs/elements/2013/10/how-l...), the owner was willing to add code tailored to tracing only Snowden's (or whoever it belonged to) account. The FBI turned him down and demanded the less surgical option.
...and a solution that would fail to preserve the chain of evidence. If there's a black box at any point, e.g. Levison produces information and emails it to the investigating officers, the doctrine of "fruit of the poisoned tree" applies.
By that logic, doesn't Lavabit itself, or the entire SMTP system, serve as a black box? It's really not any different from the intercept features in telecommunications equipment.
If the government needs to preserve the chain of evidence by controlling every step of information flow in every criminal case, well clearly the government needs to ... sniff/control/spy-on the entire Internet. Whatda-ya-know...
It's a disruption of the chain of evidence, but my understanding is that "fruit of the poisonous tree" is something different: evidence gathered because of information the police should not have had access to.
So yes, if your shopping list can't be recovered without your private SSL keys, they will take those when they have a warrant for your shopping list.