Well there is a law that they were in theory breaking.
A shitty authoritarian law, but still a law.
And the whole point of the criminal justice system is to interpret cases like these and interpret the law and decide if someone is culpable. This is what happens when you have a properly separated system where cops are not judge joury and executioner, so while cops need to have some understanding of the law, its not their job to make those finer interpretations when cases could or could not be illegal. It might seem dumb in this case, but if cops have that power it would allow them to selectively enforce the law and you would have them saying “oh I didn’t arrest the rapist because of this [nonexistent] technicality that makes it not a crime”