Wow, the number of utterly heartless, black and white thinking people on this thread is amazing.
It totally depends on a variety of factors, although I agree that 'insufficient compensation' isn't one of them.
First and foremost, is it impacting the job that you're being paid to do? If not, then who cares? Hell, if it's something interesting it may even open up an opportunity for the company to branch out into that other thing if the employe was interested in sharing that other expertise.
If it does impact the job, then the usual things come into play like past performance, corporate policies, etc. Maybe they have an important underlying reason for doing it (eg: Designing a custom 3d-printable artificial limb for a family member who lost one in an accident). Maybe they're just an asshole who thinks the rules don't apply to them and don't want to do their job. It should be a judgement call based on as many relevant factors as possible including attitude, past job performance, exceptional circumstances, etc.