Deleting the emails would have put Levison/Lavabit in violation of long-established law (destruction of evidence is a crime). Note that with PFS (if I recall correctly, Lavabit did not use PFS), preserving past emails does not help the adversary.

Deleting the account, however, seems much more viable for similar situations. Forcing a company (like Lavabit) to provide service to an individual might be supportable, based in part on past precedents to do with race discrimination and general regulation-of-business principles, but would be unprecedented in regard to a non-commercial entity. Arguably it would run afoul of the 13th Amendment in the US.

Of course, written law has little bearing on what the government can do in practice in today's USA.