In theory the IT department is correct, the screens could become virus infected, or far more likely, become part of a malware bot network. That's unavoidable in this day and age. Anti-virus software is a great comfort factor, but don't think it means a rogue application can't make it's way on board.
The screens are unlikely to become a source of infection - unless someone physically puts an infected usb stick into the unit, or the network is already infected.
There is only one real solution, keep the screens off the corporate network. Do this physically by having dedicated network cabling for the screens, or virtually with a VLAN. The IT department should be comfortable with a VLAN solution, though it's not perfect.
This is a required security measure in my opinion, not due to virus attack, but to stop people on the corporate side hacking the screens and changing messages...