This goes beyond open source, though. Someone doing this would act the same way when working in a team in a closed source project as well. I'm just saying that it's unacceptable in either case.

Open source is about fostering a spirit of sharing and collaboration. If developers don't align with those ideals, maybe open sourcing their work isn't for them.

"Open source" is abused too often these days as a marketing term, or something to fluff up your CV with, when it should be anything but that.