If I were to guess where these questions come from, it is the steep $0.02/GB ($20/TB) on overage. I think a lot of people, including myself, would prefer to buy additional transfer in bulk in advance at significantly reduced rates to avoid overage (e.g. $0.007/GB or $7/TB). I guess the question is, how much does it cost DO per TB transfer? For instance, a small $5/mo instance that suddenly gets a lot of transfer will result in a surprise bill at the end of the month. Someone might only need the $5/mo hardware option but lots of transfer for that instance (e.g. self-hosting a popular app that the person developed and published - they don't need a ton of RAM/CPU to host static content).
VPS.net has the "Power Pack" option, which goes from their default 3TB/node to "Unlimited". Whatever unlimited actually means, they don't really say and I also don't like their "temporary" additional nodes that comes with Power Pack - it's weird, gimmicky, and smelly to me. Data transfer is expensive and the server operator, not the person connecting to the box, is held responsible to pay for that. If attackers out there knew that, they would simply run up the bill by dragging out as much legitimate-looking traffic as possible over the wire without tripping any alarms.
Peace of mind is what people want. It is rather unsettling to know that someone can anonymously run up a bill because they feel like it.