I don't see the big problem with this, admittedly. If anything, the big problem is the fact that certain religions and their symbols (mainly Christianity and Catholicism) are plastered everywhere - in places where they never should be in the first place, and they hold their place in a hierarchy.
So if Satanists want to honor fallen soldiers, I don't see a problem with that. If Christians can do it, other religions should be able to do so as well; I think it's a nice gesture. This shouldn't be made out to be as something that it isn't, as in, something negative or nefarious (or as some sort of political statement, which I don't see it as such).
Perhaps the problem therein lies with the traditional viewpoint of Satanism. The biblical viewpoint, of course. If the people have a problem with it, perhaps the policy should be re-written to exclude all religious monuments. It's all or nothing and people who are of Christian or Catholic faith need to either accept other religions, or have their own excluded altogether from public spaces and from the state.
Am I making any sense at all? I'm mostly just rambling but the tldr; version is that I don't see the problem with this monument, as there should be tolerance and acceptance in public space for other faiths besides Christianity or Catholicism, because free speech and religious freedom extends beyond the ones that are cherry-picked and placed on some moral pedestal.