The problem is that these people will plagiarize their previous employers' code when working on your codebase and your code when they move on to their next employer or will also plagiarize FOSS code without worrying about the niceties of licensing. Their perceptions are one thing but the legal reality makes these sort of people hazardous to employ.
Except when their boss hired them because they're clever and can solve extant work problems (huge workload) via a happily-localized scope of concern. I've seen this easily placed back on the employer. Non-FOSS versions happen in various industries all the time.
You can make the argument that legal teams exist to relieve the very potent pressure of this uncomfortable situation for those on both ends. There are truths here that flow right up to the top and all the way to the bottom. Attacking those perspectives is too hard. Better in some ways to set up social norms around it. Thus, lawyers, the concept of win-win negotiation, etc. Just when your anger is boiling over you learn that the C-levels want you to drop the moral crusade.
