I don't think it counts as a political weapon, because I don't think that the Taliban are doing this solely (or even largely) because it will be politically divisive. I think they actually believe that allowing WHO healthcare missions into their territory will mean the CIA will come in through those healthcare missions and kill them. So that just leaves this objection: "they elected to provide greater safety to themselves at direct risk to the children".
With regards to this, not sure I can blame them, to be honest. My peoples' survival, or the survival of a handful of children somewhere, potentially far away, who I will never meet? Remember, polio isn't a big killer anymore. Even before the vaccine, mortality was declining because of improved medical care for victims. So personally, I don't think it's an easy decision to make, and condemning the Taliban for making this moral decision is qualitatively no different to calling people out about their choices in the trolley problem. The only difference is that the Taliban is actually living it, whereas the trolley problem is a hypothetical situation.