I think the situation your missing is when the employer has a much much more aggressive stance to IP. Even if you are 100% confident that your contribution doesn't violate your employer's IP, they can still sue and ruin your life.

Some employers have an unbelievably unreasonable interpretation of non-compete and IP. They think they own everything their employees do, and even though they're wrong. That doesn't stop them from ruining you and whatever unfortunate open source project they set their sights on with vexatious litigation.