P-Cat's right about Pedro's throwing schedule. As for why, I think on some level, the Sox must have felt that the 7th was the most important inning of the game -- Schilling's not available and neither is Foulke, so that makes Petey the best available option in the most important inning of your entire season. Bill James has said it before, IIRC, and I believe it's true -- you want your best pitcher pitching at the most critical junctures, whether it's the seventh or the ninth. As for the "Who's Your Daddy?" chants, I don't think you make a baseball decision based on what the crowd's chanting but on how your pitcher reacts to it. It doesn't bother Pedro apparently.
Nick