It's possible that they are just blindly antiwar (just as it is possible that other groups are blindly proanimal or proenvironment), but probably they did not march against Clinton's wars in Haiti or Yugoslavia. To understand their ilk's real motivations, it is useful to realize that the only thing they criticized Clinton for was removing the welfare entitlement. Their real enemy is private property, for all the reasons given by the socialists in the 19th century.
Cutting through the rhetoric, it usually comes down to the same 150-year-old argument--socialism versus capitalism. The Bush slander is just a tangential battle in that war.