It's not abusing a service when you're using it exactly as advertised. If you have a service that says "we'll store as much data as you can throw at us" you'd be foolish to not use it as much as possible to backup every last thing. There's nothing wrong with that to constitute using the term "abuse." Microsoft could've just as easily sold Office 365 with 1 or 2 TB of space, but they wanted to put that Unlimited text on the box.

Frankly I think they should be made to honor it.