You seem to be misrepresenting the situation. Ignorance of the law is not the defense they are invoking here.

Ignorance of the law is when you claim you didn't follow the law because you were unaware it existed.

This is a case where the court believes police did know the law and did try to follow it in good faith -- that's how they got to their conclusion that it's constitutional -- and that no court had yet reached a different conclusion in that district, until now.

These are distinctly different situations.