the difference between American Christians and Muslims in Afghanistan is not inherent in their religions

agreed

but a matter of exposure

perhaps

You certainly have a point, but I'm not persuaded that you've identified the core of the difference. My guess is that at least one other component is the context in which the exposure takes place. In America, there is a fairly strong norm to at least pay lip service to freedom of speech, and anyone growing up here who has a religion, and who has heard it mocked, will probably also have heard that this mockery is defended by another national norm. Exposure to equivalent mockery in Afghanistan may not have equivalent effects.