Here's why it's so hard for the government to bust Herbalife

For the past three years, activist investor Bill Ackman has criticized the nutritional supplement company Herbalife to anyone who will listen — and on April 27, his audience happened to be a group of U.S. senators. “Herbalife is causing enormous harm” to the people who sign up as sellers for the company, he said, even though he’d been summoned to Capitol Hill so legislators could grill him on an entirely unrelated topic—his investment in the troubled drug company Valeant Pharmaceutic...