I didn’t know either, I thought the Mass was going to make the building impure by Halakhah standards. Also, it is hard to imagine observant Jews celebrating Passover in a building over a tomb, but even if we accept that, it still seems to me like it is the tomb of Israel’s king, in Israel’s capital... I didn’t feel comfortable with having a Mass there. Aren’t there plenty of other holy sites in Jerusalem?
Yes, that's exactly the problem--rendering the Tomb of King David ritually impure. But of course Catholicism doesn't recognize Halakhah. I can understand why they wanted to have a mass there . . . it's supposed to be the exact room where the very first mass was said. And at the time, I don't think there was a tomb underneath it (I'm not sure, but I think David's body was brought there later from its original burial place).
This is simply a case of one religion being right and the other being wrong--or one true and the other false. American Fundamentalist chrstians don't like to think of it this way, but that's the way it actually is.