USB access by definition is a pathway to introduce malware, spyware, viruses both intentional and unintentionally, even boot a separate operating system then mounting the host device to access its files. Once a USB is then separated, it can be substituted or edited in another device or have malware, spyware, viruses introduced during transport.
How many people had the knowledge of the encryption keys? Encryption is not the end all when physical access is possible and especially so when the encryption keys could be compromised.
Well, you need to pick your poison. Connecting the tabulators to another system offers the same chance for compromise.
It seems the safest approach might be to have separate USB drives for each tabulator so a compromised USB drive could only infect one machine.