wow bonus is a ****

I have a prepaid sim and ever so often I qualify for the wow bonus (use R11 airtime and get R100 for mtn to mtn calls plus 20mb data). I used it in the past and found that it was generally to use up your available airtime. I ignored this 'bonus' until I tried again on Thursday 18 June. I had about R11 airtime when my wow bonus kicked in. U monitored the usage wanting to stop at 16mb. Usage went slowly until 12mb and then was suddenly on 36mb using up my airtime. On Friday, I tried again. I was downloading emails. The download showed the emails being downloaded was 5mb which is quite accurate. After downloading, I checked my usage and found 32mb was used and my airtime of R12.60 was gone. I have nothing that runs automatically so nothing else was downloading. I normally use cell c for data as they are the cheapest and very efficient and accurate with usage statistics. The only reason for using the wow bonus is that it was freely available, I have very little to download and don't want to recharge my cell c until the morning because they give you real bonus. I have clearly been ripped of by mtn by misleading information regarding this stupid wow nonus.