Mtn
BlackBerry now limited, was not informed

I recently changed from contract to Pay as you go on MTN.
I bought the full Blackberry service.
I now get an sms stating that my "fair share" of blackberry service is finnished and I now will be charged for further data.
When I purchased this I was not told or informed in any way.
Why is it not cheaper then if it is limited?

Disgraceful service from an expensive careless company.