I guess I am trying to figure out just in what context displaying the ten commandments could be considered a historical display.
American law is basically a continuation of English law, which owes its historical origins to old Germanic customs and Roman laws, both being pagan in origin. The same can be said for continental European law, except with more emphasis on Roman precedents.
The ten commandments are a relatively minor portion of ancient Jewish Law, which had more to do with the ritual separation of the Hebrew people from the pagan tribes around them than with a codification of civil law.