Why would the parties to the ACTA negotiations be reluctant to disclose the details of the provisions under consideration?

In 2008, the United States began negotiating a new international treaty with the European Union, Japan, and Switzerland. By 2009, Australia, Canada, Jordan, Mexico, Morocco, New Zealand, South Korea, and the United Arab Emirates had joined the negotiations. The treaty, called the Anti-Counterfeiting Trade Agreement (ACTA), will establish its own governing body that is separate and distinct from existing organizations, such as the World Trade Organization and the World Intellectual Property Organization.