UPnP is a technology to allow auto-configuration of NAT/Firewall enabled devices. If you have an appropriately configured network and allow the required traffic through the firewalls, there should be no reason that UPnP is required. From an Operations-Security stand-point, I would highly recommend that you put them on their own VLAN.
I think one of the reasons why they are asking for a separate network is to ensure compatibility. Not everyone can manage a firewall, or create a VLAN, and it's easier for them from a support perspective to just say they don't support it. It may also have to do with the bandwidth required to properly run the devices. If the switches they are going through are already pushing lots of data, they might not get enough throughput to properly display signals without some type of loss or degradation.