It seems like many business branded comps (Dell, Lenovo, etc) have security issues on onboard ethernet (exposing either IPMI or AMT). On my router machine, I'm only using the onboard ethernet for a 'management lan' that gets no internet access at all. IPMI and switch management interfaces shouldn't need to access the internet anyway. WAN and LANs served by an intel 4-port card. I just don't trust those manufacturers to patch management bugs fast enough.
For all those who ran into this data usage issue, I'd urge you to reset your IPMI or AMT interface to factory defaults before locking it down. It's an annoyingly nontrivial task in some cases. If it's got gigs of data usage, I'd be concerned that a bad actor somewhere has pwned your management interface.