The answer is fairly simple, the vast majority of users will go nowhere near the 1TB limit, so they're subsidising the more hungry users, the people who pay the extra £9.99 a month are likely to use the whole Terabyte eventually, so are carrying the full cost.
We have around 1,100 One Drive users at work, on average they're using 100GB of space, and 700 of those users are students who pack their folders with all kinds of crap we'd rather not know is there...
Biting the hand that feeds IT
