'Civilian control of the military' is incidental to this argument. Any civilian posting criticism of their employer's senior executives and/or company strategy on social media could easily find themselves on the wrong end of an internal disciplinary process and potentially be dismissed for bringing the firm into disrepute.
FWIW I think he was extremely unwise to speak out in this way. He is far, far too low down the food chain to make any kind of difference: 3* resignations can easily be glossed over, potentially even 4* if the politicians are prepared to play dirty. There are many public calls for accountability already being made and one lowly O-5 is not going to make a scrap of difference to that debate. And holding the moral high ground won't pay the bills.
Better to have resigned from the service with a polite but hard-hitting private letter explaining loss of confidence in the chain of command. VIP egos are such that attacking them in public invites a counter-attack which sweeps away the complaint as well as the complainer. Pricking VIP consciences in private might have a low probability of success but at least it's above zero.