MTN theft of legally purchased data.

I have a pre-paid MTN phone. No contract. Obviously I have to bye air time and data. when purchasing data, every bundle that only last for a specific time, indicates on the bundle you purchase for how long it is valid, however when purchasing a super data bundle it gives you the option for "once off purchase" no indication of a expiry date in any form. Now, I bought a 30G super Data bundle, price R269.00. In 30 days time, whatever is remaining on the bundle is taken back by MTN and I do not have access to it anymore. I queried it with MTN and I was informed that if I purchase a new bundle, the remaining data of the first purchase will be available for me again. this was an either completely an answer to solicit me just to purchase another bundle or an totally unknowledgeable person. when I purchased a 30G super data bundle, I legally paid for it,30G. every time after 30 days, MTN takes back my remaining data which I paid for, thus I did not get the value for what I paid, my property. I see this as THEFT from MTN as I paid for a product that is then taken away without my consent. My only expectation is to use the item (data) I purchased legally and without complain from being *****ing by the service provider.