Chelsea Manning was imprisoned over something very different: she was a US soldier, not a private citizen, and she made public internal government documents, not a letter explicitly addressed to her.
Different standards do (and, IMO, should) apply to leakers who are government employees or members of the armed forces with a security clearance and to private citizens who are not 'leaking' something but rather sharing something they did not explicitly and voluntarily (through a contract or NDA) agree not to disclose.