No one should. It's quite clear that all three religions are attempting to worship the same God. They all came out of Judaism. I suppose some Christians may take issue because of Christ also being God, but in fairness it should still be the Father that Christians are worshiping. Even if Christ is the living embodiment of God, the language of the New Testament makes it quite clear that his job is to "intercede" for us. Christians are to pray to God through Jesus, as an intermediary, but still they pray to God. Christ could be seen as being God's mouthpiece to us, and ours to God, in a similar way that Aaron was Moses' mouthpiece to the Egyptians.
It's certainly fair for anyone to feel the other group is worshiping God in error, but to disagree that all are trying to worship the same God is ignorance.