These on-site inspections, often four a year, are unannounced. Lab inspectors can require manufacturers to present data and to rerun safety trials and experiments. Companies, in turn, pay a tiny fee for every UL designation symbol they put on their products. Manufacturers can modify their products to adapt to market conditions, but the Lab oversees changes that affect product safety. The Lab is inflexible and scientific, but it's also driven by common sense and realism.
Nothing is perfectly safe, of course.
The competitive marketplace and the Lab aim for safety in a framework of rational attention to costs. In fact, the Lab once built a fireproof office for some of its employees. The expensive room featured ceramic tile on the walls and ceiling, a thick concrete floor, metal furniture, and similar standards. Not only was the cost high, the esthetic results were not impressive. It takes this into account in its requirements.