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.
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.
(Erm…disclaimer: IANAL)