GUEST OP-ED CONTRIBUTOR Los Angeles Say that an I.R.S. agent leaks a politician's income tax return to a newspaper reporter, an act that is a federal felony. The newspaper may have a First Amendment right to publish the information, especially since it bears on a matter of public interest. The government, meanwhile, is entitled to punish the agent, to protect citizens' privacy and ensure a fair and efficient tax system. To punish the agent, prosecutors may need to get the leaker's name from the reporter; but if the reporter refuses to testify because of a "journalist's privilege" to protect confidential...