Please review the following response which explores the difference between voluntary and mandatory labeling: http://gmoanswers.com/ask/regardless-whether-or-not-you-believe-gmos-are-good-or-bad-what-harm-labeling-them-so-consumers.
One passage addressing your question states:
“As believers in GM technology, and having seen the benefits nurture farmers and society alike (Check out GMOs and the Future of Agriculture). The harm comes from a label that conveys to consumers that food made from farmers’ crops grown with our seeds is somehow less safe or nutritious or somehow different from conventional or organic food. This is simply not the case. We believe a government requirement to label a food ‘GMO’ would do just this, and a recent study conducted by an MIT professor supports this view [See Policy and Inference: The Case of Product Labeling].”