Balderdash! These Marine Generals were not sacked because of any failure to say whether they would or would not "take up arms against Americans." They were sacked because of a failure of judgment in combat to take standard, prudent steps required in a combat zone to see that your position is not infiltrated, your men and materiel unnecessarily put at more risk. As a commander, you are responsible for everything that your command does or fails to do. If your staff failed, you failed. If your subordinate commanders fail, you have failed. It has always been this way, and needs to stay that way. The loss of a dozen Marines and an entire squadron of AV-8B Harriers was inexcusable under the circumstance in which it occurred. It was foreseeable, preventable with basic fundamental practices, and should not have happened.
By the way, the oath of office includes the phrase "against all enemies, foreign and domestic." That phrase has never excluded Americans from being in a group against whom any oath taker might be asked to lawfully take up arms.