The courts accept good faith arguments at times. They will give reduced sentences or even none at all if they think you acted in good faith. There are enough situations where it is legal to kill someone that there are laws to make it clear that is a legal situation where one person can kill another (hopefully they never apply to you).
Note that this case is not about ignorance of the law. This is I knew the law and was trying to follow it - I just honestly thought it didn't apply because of some tricky situation that isn't 100% clear.
The difference between "I don't know" and "I thought it worked like this" is purely a matter of degrees of ignorance. It sounds like the cops were ignorant of the law in the same way as someone who is completely unaware of it, just to a lesser degree. Unless they were misinformed about the origins of what they were looking at, it doesn't seem like it would be a matter of good faith, but purely negligence.
Note that this case is not about ignorance of the law. This is I knew the law and was trying to follow it - I just honestly thought it didn't apply because of some tricky situation that isn't 100% clear.