Because we hold freedom of expression, freedom of religious belief (or lack of), and (within limits) freedom of speech as important freedoms. So we should apply then to others because we believe it's right and not conditional on whether they apply then to us.
Do to others as you would like to done to yourself, as a lack of religious tolerance because (and if) someone doesn't tolerate our religion hardly speaks well for how we practice what we say we believe in, does it?