For one thing its a way to make money;; what you have paid for is not very useful, what you will or is now paying for is useful to the company.
That is the most basic answer from any business point of view.
Why dont you take up the post paid ? It has worked for me,, i.e pay for only what you use ??
Someone informs me that there is an orange data advert on todays paper,,,i am yet to see one to confirm the exact page,, mabbe you could have a look ??