“Why don’t we let Western European countries, which Poland would so much like to have a strong relationship with, convince Poland to do this voluntarily?”
And which Western European countries would that be ?
Poland, like most Eastern European countries, would love to be accepted into the "European community" by countries like France, Germany, the UK, Belgium, etc. If those countries are interested in a missle shield that can protect them, I'm sure they would have no problem persuading Poland to help with it.
If they don't want to help, is there any reason the shield actually needs to include Europe?