MTN *****s airtime to convert to data

A while ago I had the issue that though I have mobile data available, MTN chooses to deduct airtime to convert to data access. I raised the issue with them and they give me a proforma response, that I have allowed out of data to be used. When I check I find that is not true, but they deduct it anyway. Anyway for a while now I have been fortunate that I have not needed to use mobile data. Now today, once again I had to make use of mobile data. i had airtime and data available. MTN first used all my airtime, and only then started to use my data that was available. I raise the issue again with them and they respond by when did it happen. The only explanation I have for this is that, MTN can make more money *****ing your airtime to convert to data, than allowing you to use data.