Its not uncommon to want a short-term loss for your loved one with a hope for a long-term gain. If mom calls police on her son trying to straighten him up; or you cheer against your home team with the hope that they will finally fire that bustard and the days of old glory will return with a new guy. You see what I mean? So, yes, I can imagine how a leftists can be confused (oxymoron, isn't it?) into thinking that they do support our troops but are against the war. It is entirely different thing to actually believe them ...
Personally I think it's just PR. An attempt to combat the bad PR they got during the Viet-Nam war.