It's only an issue because they measure data consumption as a part of their billing infrastructure, and to determine if subscribers have exceeded their monthly allotment. Their ToS also prohibits the use of their non-commercial services for commercial purposes. So, I think they're within their legal right to investigate my data useage if I exceed certain thresholds, which I most definitely did. 

An individual subscriber who uses nearly a terabyte of data in one month is bound to raise questions. They weren't interested in the details of my use, only that I was only using the service for personal use. I chose to establish that by describing the devices I was using, and attesting to the fact that I was using the data exclusively for personal use. That's as far as it went. I invited them to contact me if they needed any further information, and they never did. So, it's case closed.

In general, I don't think they do this regularly because I've asked all my Verizon friends and family if they've ever been investigated for their data use, and not a single person had. So, I was an outlier due to my admittedly unusual data consumption.