What would be a better system?
Combine this with a PKI and you should then have a solid solution. If you want maximum traceability, you could throw in LTE or satellite connectivity for logging major audit events or triggering certain device features remotely.
The police can and should be held to a higher standard regarding evidence and chain of custody. The tools are only half of this picture. It is a synergistic approach.
I'm curious as to who is responsible for not properly erasing the files: the IT department, or the manufacturer. It could be that the station didn't wipe/reset the cameras before selling them. Or they only reinitialized the file system. Or the firmware's reset function only reinitializes the file system rather than shredding the contents.
I can only imagine that the information security story around all of this kind of hardware is probably pretty awful.