The primary reasoning behind the civil tort of negligence is that those who fail to act in a reasonably prudent manner should be held accountable to those hurt by their actions (or inactions).
Just as direct negligence can be used to hold a careless individual liable in court, vicarious liability (sometimes known as respondeat superior) can be used to hold an employer responsible for the conduct of an employee.
Of course, not every action by a worker amounts to liability for the company for which he or she works. A recent case explored the law in Texas with respect to this issue.