IIRC, they did not store user's keys. What they wanted was for Lavabit to turn over their SSL key, which would have compromised security for the entire service, rather than just targeting Snowden specifically.
Presumably they did not have the ability to turn over the emails themselves, so they wanted Lavabit to provide a vector to do a man in the middle attack.
Presumably they did not have the ability to turn over the emails themselves, so they wanted Lavabit to provide a vector to do a man in the middle attack.