Legislative Privilege?

I’m not a constitutional law expert, but this seems strange to me:

Jefferson argued that the first-of-its-kind raid trampled congressional independence. The Constitution prohibits the executive branch from using its law enforcement powers to interfere with the lawmaking process. The Justice Department said that declaring the search unconstitutional would essentially prohibit the FBI from ever looking at a lawmaker’s documents.

The U.S. Court of Appeals for the District of Columbia Circuit rejected that claim. The court held that, while the search itself was constitutional, FBI agents crossed the line when they viewed every record in the office without giving Jefferson the chance to argue that some documents involved legislative business.

How does that work? What’s to keep the Congressman from arguing that all the records had to do with legislative business, and not allowing them to see anything? The real issue here, since they at least ruled the raid itself illegal, is whether or not the trial judge will throw out the untainted evidence.

I wonder if Justice will appeal?