Employers generally have the right to discipline their employees for actions that are contrary to the rules of the firm. In the case of internet abuse, the employee should, in general, not be punished unless the firm has an internet usage policy in place before the employee misused the internet.
Employers should not assume that they can discipline employees for anything that they deem to be improper after the fact. They should be sure to have regulations that explicitly state what is punishable and what is not. There are some exceptions. If, for example, the employee is doing something illegal with the computer (trying to conduct scams, for instance), the employer ought to be able to terminate the employee even in the absence of a preexisting policy.
However, for more pedestrian internet misuse, a firm should not be able to terminate or otherwise punish an employee unless there is a clear-cut policy that covers the employee’s actions. If such a policy does exist, I am in favor of terminating employees who break it. It is an employee’s responsibility to follow the rules of the place where they choose to work.