He didn't choose to keep it open source. He has no choice unless he tracks down all the contributors back to B2 and gets them to assign copyright. This would be

He didn't choose to keep it open source. He has no choice unless he tracks down all the contributors back to B2 and gets them to assign copyright. This would be the most controversial thing he's done so far and likely lead to a fork. That's the point of a license like GPL.