Data Purchase and expiry time

Any time I buy data for my mobile modem I pay the exorbitant fee and then get told I only have a limited time period in which to use the Data otherwise it expires. Why is this allowed after all I have paid for a certain amount of product I should be free to use as I want.<br> After all if I buy a tin of beans from Pick n' Pay they do not request that I return them just because they sit in my larder for over a month.<br> <br> I am really thinking about starting some sort of campaign against the cell operators in South Africa, not only do we get ripped of at the cost compared to other countries, they have the ordasity to take it back after I have PAID FOR IT.<br> <br> Please assist or advise,<br> Alan.<br>