Because we are dealing with a generally decentralized enemy here.
By telling what we know of their concrete plans, the decentralized cells tasked with carrying out the now-public operations will be forced to break their radio silence.
They will have to communicate back to Osama, Zawahiri, Zarqawi, or Mullah Omar for new instructions.
When they communicate, they are vulnerable. Intercepting signals is one of the few intelligence tricks that we Americans are good, no, *exceptional* at doing.
So to round up terror cells that may already be in place here in the states, we've got to convince them to break their "radio" silence. When they are silent, we've got precious little chance of catching them prior to their attacks. When they talk, not only can we track them, but we also gain clues about the locations of their leaders.
5 Legislative Days Left Until The AWB Expires
Can you imagine how far ahead we would be if Clinton had put an emphasis on the training of Arabic translators?
I think that was one of the anti-terror suggestions contained in the stolen Millenium report.