Unfair billing practice

I'm so annoyed with mtn. Last night I was trying to use a certain app on my phone but what happened was instead of my data being used for that Internet connection it chowed my airtime instead. Why is that, I mean the reason I buy airtime and data is simply because I don't want to use airtime for Internet as it gets consumed too quickly but with mtn they rather go airtime than data. I'm so angry at this moment I'm even afraid to load airtime to my phone as I might get the 0.00 surprise of my life.