Sensitive But Unclassified or How to Get Around FOIA

Here is the CDC's document telling their staff how to with hold information from the public that was referred to in a previous post:

http://www.fas.org/sgp/othergov/cdc-sbu.pdf

"The purpose of this document is to provide policy and procedures to the Centers for Disease Control and Prevention[1] (CDC) that allow for the accomplishment of our public health service mission while safeguarding the various categories of unclassified data and document information that, for legitimate government purposes and good reason, shall be withheld from distribution or to which access shall be denied or restricted."

I thought that their "health service mission" was to gather and disseminate health information?

The Sensitive But Unclassified label "is applied to unclassified information that may be exempt from mandatory release to the public under FOIA."

So I want to know who decides what information is able to be kept from the public? If it truly needs to be kept from the public, then why is it not given a "classified" status?

What measures are in place to prevent the CDC from with holding information from the public that could implicate the CDC itself in poor public policy decisions or downright fraud?

Poor public policy decisions and fraud are things that have actually happened once or twice before in government agencies, and I have even heard of people in government agencies trying to "cover up" such problems by "withholding" information from the "public".

This policy sucks.