As has already been pointed out, it's his use of it that is the problem. Further, I am not sure that it would ever be sensible for the Commander in Chief to effectively dismiss the death of a member of the military as 'they knew what they signed up for'
Leaving Trump's use of it aside, I am not convinced by the idea that he is correct or that it is true. I've seen a couple of pieces that suggest that the death of David Johnson related to a number of cockups, in particularly that the rescue helicopter was a non military contractor and that there was no check of numbers which lead to him being left behind in error while still alive. While one has to wary of false news, if it is the case then he didn't sign up for that.
In addition, there is a current campaign to stop people being signed up to the army at 18, and there is the idea that Gordon Gentle who was killed at 19 wasn't perhaps as sure about what he signed up, especially the lack of proper equipment that contributed to his death. The whole 'Join the Army, See the World' or get a trade approach has been used to offer a chance for many kids who have limited opportunities to think it is much more glamorous and much less dangerous.
I am sure there are many in the military who have carried out a rational cost benefit analysis of what they have signed up for but no one signs up for incompetence, no one signs up for useless kit, and no one signs up for their commander in chief being so crass as to make a grieving mother's grief worse. As a pacifist, it can be too easy to see joining the military as some kind of claim to violence, but many of them are barely more than kids, given a chance to escape a life with little hope, badly equipped to evaluate the likelihood of being badly equipped. We like our cannonfodder.
