I believe it ends up getting enforced at the retailer level; most major chains will refuse to purchase Christmas lights that are not UL listed, so you can be reasonably sure that the cheaply manufactured lights you purchase at Walmart don't burn your house down. They also seem to be providing education [1] and revising their standards [2] to discourage particularly ill-advised appliances such as electric turkey fryers.
[1]: http://www.ul.com/global/eng/pages/offerings/perspectives/co... [2]: http://ulstandardsinfonet.ul.com/scopes/1083.html