I agree with your post, but this part is kind of wishy-washy.

> I don't get how knowing that they could be ordered to do something legal-but-blatantly-political means that they should have expected that eventuality.

Most folks who are in the military or are considering it have heard of the honor guard. This is the most obviously political post one can have, but it is arguably one of the most important, due to the virtues such a post embodies, and the highly visible, public nature of the post.

Many folks would leap out of their seat to have such a post, though I can see how some would rather decline if given the option, due to the importance of the job and perhaps their own feelings of unsuitability, or desire to not interact with the public, or whatever.

I think it's an inherently political job, and everyone should know that going in. What you do in uniform reflects directly on the nation whose flag your uniform is emblazoned with.