You say "Poland and the Czech Republic sought the system in order to secure U.S. support in the face of recent Russian assertiveness"

My understanding was that this was a U.S. idea, not something that the Poles or Czechs "sought." The deployment is very unpopular in both countries. You are correct in the sense that their focus is on Russia and not Iran. Poland and the Czech Republic accepted the U.S. proposal in order to secure a firmer military commitment from the United States. Before agreeing to it, however, the Poles, in particular, demanded U.S. assistance in upgrading their military and also the deployment of a U.S. Patriot battery to cover them against any adverse Russian reaction to the BMD deployment. (They've been saying lately that the Patriot commitment still holds even if the BMD plan falls through.) The U.S. would not agree to this until after the Russian invasion of Georgia (although most of the news reporting implied it was the Poles who changed their position after the invasion). If the Poles and Czechs are mad now, it will be because the governments had exposed themselves politically at home for something the United States wanted. That, however, is not reason enough to follow through with a bad plan. We will just have to find other ways to make it up to them.