...One reason, as many suggested, could be that the employee might not have been fairly compensated despite being exceptionally good at the job. In which case, the problem resides somewhere in the management who has failed to live up to the expectations.
What sort of clap-trap is this excuse. If an employee tries to use it, I'd consider it more evidence that the employee should be fired.
Bottom line: if you don't like the management of the company, then leave. If you are doing side work on the job, that is the equivalent of goofing off.
Quit your whining and get back to working the job you are being paid for.