Some services are more important than others. If teachers go on strike kids miss some school; if nurses go on strike people die. The military fits into the "people die" category in terms of likely consequences from a strike.
Edit: just to be clear, I don't mean to imply that teachers aren't important, just that the immediate consequences of them all missing work is less severe than some other occupations.
Do you really think that more people would die if the US military went on strike? Besides, the US military already has a mechanism to force people to work for the military.
My original comment was about the Canadian military and was an example of how groups like the police don't necessarily need unions so you're the one bringing the US military into this. I can't speak for the US military but the Canadian military is active domestically doing search and rescue, responding to natural disasters, counter terrorism, etc. People will die if those activities need to take place and the military is on strike. They doesn't account for other important work like embassy staff. I know the US military provides embassy security. How many people would die if no US embassies had armed people with guns protecting them?
It's also important to consider that when you're talking about the military who dies matters more than how many people die. How many civilian drone strike casualties would it take to get the same attention that Benghazi got?
What mechanism does the US military have to force people to work in the event that the whole military is on strike? Are they going to charge the entire military if they go on strike? Everybody walks down into the brig and the last man in locks the door behind them?
Draftees won't be much help if the military is on strike. Let's take a quick look at how that would go.
A group of young adults gets drafted into the Army. They're told to show up at base Foo on some date for boot camp. They go to base Foo and cross the picket line to enter the base. All of the instructors are on strike so there is no one to teach them. They can't get uniforms and equipment because the quartermaster is also on strike. They head to the mess hall to get some food but the cooks are on strike. It's getting late and they want to sleep. The barracks for trainees is locked up and no one is around with the key because they're on strike.
Now you've got a group of tired, hungry people, who didn't want to be there in the first place, who have no training or equipment. How does this help?
What’s unique here? Isn’t the whole point of all strikes that the employer doesn’t want the employees to go on strike?