Looks like a matter of perspective to me.
Asfaik, it's a tenet of Judaism, Christianity, Islam and Baha'i faith that there is only one God, and no god besides God. Hence these religions are called "monotheistic".
When you're a believer of either of these religions, it would thus be "idolatry" if you claimed there was more than just this one God, you'd contradict the most basic tenet of your religion.. If you're Christian, that leaves only two possibilities: Either the Jewish God was not a God (but a mere unreal figment of imagination, a lesser spirit, etc), or it is the same one God Christians are worshipping.
The other view would be the distanced, irreligious view: Gods exist only in the minds of the believers. Hence, when believers of different religions ascribe different attributes to their respective God, they're different Gods. Because in this irreligious view, God is not real, but merely man's different ideas of God. According to this view, Jews worship a different God than Christians, since their theology is different.