I had the same plan/setup, using a Nighthawk with an iPad plan. It sure worked great, better than my Verizon service most of the time. But that 450GB of data we used back in July was too much to ignore, I guess.
They sent the same cancellation notice, mentioning the fraud dept. But no one could actually tell me why it was cancelled. My sone got set up on the same plan and his is still going strong, but he doesn't abuse the data like me!
No, I would not blame the guy I bought from on EBay..he disclosed it was an iPad plan and could be cancelled. I knew this and won't blame others. I bought at my own risk...such is life.
I've since given up on my 20 + years on Verizon due to weak signal/data, etc. AT&T is stronger in my experience. But I know AT&T is a scuzzy outfit...money-mongers! But I want the data at the right price and because they are so careless, we can take advantage of this with their various plans they don't always monitor for abuse. My next plan is to add a $20 iPad plan and tether to a router. So with my two phones and that Pad I should have 300GB available before throttle...and if I'm correct they may never get around to slowing me down on all devices at the same time.