At the end of the day it doesn't matter how you define "MITM". HTTPS means that even if I control a network hop in between the two endpoints, I can't get the key. Provider B says "we are OK with any hop on your route gaining access to your keys" (e.g. via owning a wifi router and giving customers access, or via being a customer and sniffing other customer's traffic, or the industrial-level equivalents of this).