Modern HVAC controls are much more than thermostats. There are typically resets for supply air temperatures based on outside air conditions and time of day, and boiler water temperature setbacks based outside air conditions. Fan and pump systems can get feedback from the positions of dampers/valves throughout the system, and the VFD can slow down to minimize energy usage based on the feedback from the worst-case zone in real time. The list goes on, but all of this energy optimizing relies on lots of real time data, and the easiest way to do this is on an ethernet network.
Many large clients, particularly those with multiple locations like school districts or big box stores will hire a controls company, and pay them a bunch of money to save a target dollar amount or percentage amount on their energy costs. This is typically done through an online interface to monitor multiple locations simultaneously, and keep them all operating the same way. The user doesn't typically care how the contractor sets this up, they just want the savings. The cheaper the contractor can get to the target the more money he makes, which can lead to corner cutting by the contractor.
Some people (government, some Universities) tend to make the controls sub-contractors install a second, independent TCP/IP network for their equipment. But this security comes at a cost premium, particularly in existing buildings that already have a network in place for their computer needs. Most places I have seen don't bother with this due to the cost and the general availability of network connections in today's world. If the security is setup properly this shouldn't be needed, but we all know how often proper security is overlooked.