They did not affirm that the First Amendment was not violated; they explicitly ruled it had. What they affirmed was that it was not so obviously violated that every officer should have known it, and therefore the officers were entitled to qualified immunity, i.e. immunity from being sued because they did their jobs in a way that a reasonable officer could think was lawful.
In the US, the law is a combination of the written letter of the law and the interpretation courts have given to that letter via precedent. This extends to the Constitution. If there aren't rulings about an issue close enough to the one at hand, then the law is, in fact, unknowable.
In addition, qualified immunity applies to civil suits, not criminal cases. If a prosecutor had deemed the officers' behavior worth prosecuting as a crime, qualified immunity would not help them (though I don't know if something equivalent exists for criminal offenses).
To recap, qualified immunity gives government officials cover from civil actions when the law is unknowable.
But isn't the practical effect of this that police officers can continue violating the FA rights of people in this way and then shrugging behind qualified immunity?
No, because they have firmly established that right by this very ruling.
"Although the right was not clearly established at the time of Turner’s activities, whether such a right exists and is protected by the First Amendment presents a separate and distinct question. Because the issue continues to
arise in the qualified immunity context, we now proceed to determine it for the future. We conclude that First Amendment principles, controlling authority, and persuasive precedent demonstrate that a First Amendment right to record the police does exist, subject only to reasonable time, place, and manner restrictions. "
