Teixeira took an oath to defend the US as well as its military secrets — secrets that are vital to US national security and the physical security of personnel serving overseas. In breaking that oath, doing so almost daily for a span of a year, Teixeira committed one of the biggest violations of the Espionage Act in American history. His actions demand strong consequences, both for himself and as a message to other potential leakers.
While Teixeira's actions were wrong and misguided, they were not motivated by malice but rather by a desire to connect with friends he had made online, borne out of his autism. He is a youthful offender who has his whole life to look forward to. A lesser sentence would be more than sufficient for him to mature and pay the price for what he did — responsibility for which he has already acknowledged.