They'd likely argue that forcing them to sign non-Apple software as if it was their own is compelled speech.
If there were true rule of the law and everyone would have been treated equally in US, then either both Lavabit and Apple should give out signing keys, or none at all.
Lavabit was forced and had no choice. As far as my reading and understanding goes, the Gov created some sort of loophole where Lavabit was denied a hearing and because of lack of hearing he was... found guilty (catch 22). I'm sure others can shed more light...
At this point, Lavabit could've complied by just handing over the original data, but instead decided to get cute. "Contempt of court" is an aptly named crime.
That may be. However, simply forcing them to hand over the key wouldn't imho.