Based on the "no interest for X months" credit model by the sound of it. If that loan isn't paid off before a certain date, they get you on some stupid high interest rate for the entire amount & duration.
They're 100% betting users will over run because that's what they do.
I don't use iDrive. Does the service/application inform the user that they're over quota, or close to it- or does it quietly over run?