Muslims in Pakistan are nourished on hatred. Hatred for the "occupying" Indians in Kashmir, hatred for the Jews and their tiny nation, hatred for the Americans who fight the true and pure Muslims, hatred for the infidels everywhere.
In fact, in their minds, non-Muslims are utterly deprave and wretched; they are incapable of good, moral behaviour and charity towards Muslims. For you see, infidels hate Muslims and they're out to keep the followers of Islam under their heels. This means that every act by a non-Muslim person or a nation is suspect. The narrative is simple: infidels are evil.
So, when young, naive, generous and kind non-Muslims travel to Islamic lands and use their precious time to provide polio vaccines for kids, the Muslims, of course, come up with a diabolical motive. The vaccines are not for eradicating polio but merely an excuse for making the kids infertile! Don't you see: the Muslim population growth rate far exceeds the rate for infidels. This "vaccine" is just their genius plan to limit the numbers of the moral Muslims of the world.
For Muslims, what's the logical answer to these altruistic people who want to vaccinate children? Death threats.