Sorry, I cannot agree with some of the posts above. It is not political correctness, it is the result of horrendous behavior by many individuals.
There are people out there with libraries of pictures of naked children who also engage in horrible physical and sexual abuse of children.
The "zero tolerance" rules are there to take the discretion out of the hands of people like store clerks and lab employees.
The discretion and judgment is quite rightly transferred to people who are knowledgeable about the scourge of child abuse.
The problem with the OP's situation isn't with the lab, it is with the police, who followed up, determined that there was no problem, but confiscated the entirely innocent slide anyways.
The police may have been forced to confiscate the slide, due to the rules they are required to enforce. If so, those rules need reviewing.