“from the photos it appears the stones are in pretty good shape. Why was it necessary to replace them”
Those are horizontal markers that lay flat on the ground. The last time I was at a National Veterans’ cemetery, all the gravestones were the vertical type.
It is possible the style is being changed from one to the other, and the stones replaced regardless of condition.
Even if this homeowner came by these legitimately, this seems quite disrespectful.