We need more money because the FBI already has this power and the staff to do the investigations! We need our own duplicate empire because ... because we do.
It's worse then that. It appears to me that Rumsfeld and Cheney have an intentionally policy of shifting as much of the authority and budget as they can to the Pentagon and putting it military command.
A lot of this is grounded in bureaucratic turf wars, but I think Cheney and Rumsfeld have a deeper plan. They seem to be planning to bring as much of the government as possible into the military, which results in less legal restrictions, less oversight, more direct command and the militarization of what should be civilian matters.
Jay