Corporate policies can miss their targets. If a policy targeting the slackers existed, than part of it's job would be to target said slackers, which it would only be able to successfully do with some probability. However, if there isn't a statistically significant number of slackers, the policy could end up falsely targeting more hardworking employees than slackers. It's a classic probability/confusion matrix problem and goes to show that you have to be damn sure that your policy is only targeting the right people.

A company willing to risk harming its honest employees for minimal benefit is not one I would like to work for.