It's actually not legally fine, or at least it's extremely dangerous. Projects that re-implement APIs presented by extremely litigious companies specifically do not allow people who, for instance, have seen the proprietary source code to then work on the project.

I don't think fear or legal action makes it illegal.

If I know it is legal to make a turn at a red light. And I know a court will uphold that I was in the right but a police officer will fine me regardless and I would need to go to actually pursue some legal remedy I'm unlikely to do it regardless of whether it is legal because it is expensive, if not in money but time.

In the case of copyright lawsuits they are notoriously expensive and long so even if a court would eventually deem it fine, why take the chance.

My understanding is that that is a maximalist position for the avoidance of risk, and is sufficient but probably not necessary.