The problem is that a report about a phone scanned on 2020-02-01 can be altered by a phone scanned on 2020-05-01 to say that there was porn when there wasn't. Oh, and that scan left a running program which will cause 5% of the phones scanned after that to randomly also claim porn that is not on the device.
Therefore if a single phone with Signal was scanned at the kiosk, NOTHING from that kiosk can be trusted.
If the USER could select the action, for research purposes, that might a different story.
If he winds up in court, I'd love it if he sticks to his, "the files are there for artistic effect".
Saying those decorative files tampered with evidence is equivalent to admitting that everything the Cellebrite claims to do, it doesn't do and never has done.
In absence of write-once media, they are betting that the hashes they capture will be the same every time they image the device because they never modify the device.