@khm this is a question for the software freedom conservancy. Their site might give you some relevant stories.
Copyleft is just as hard to enforce as copyright, generally, unless you can afford a legal team willing to challenge larger legal teams. Varies by your locality, but you're right to think code theft is the norm. Big legal departments just play cleanup for software engineering teams that are simply told to meet a deadline at whatever cost. This is true regardless of licensing, or how the code was shared.