Ummm, isn't that PRECISELY the point? If the search criteria isn't sufficiently broad to catch someone then that means they don't have enough evidence to be conducting the search in the first place. Almost everyone can be found guilty of some illegal activity (however minor) if the search parameters are sufficiently broad.
Genuine question. If I employed the services of a company specializing in archiving paperwork, and the government had a search warrant for potential evidence in their case which could be contained in that paperwork, wouldn't the prosecutor (or at least someone working under the prosecutor's direction) be the one looking through it? As the argument goes so many times against the government's practices, why should we expect that, in the case of email, searches should operate substantially differently than with paper records? In this case, it seems wholly appropriate to apply it in the other direction.