Free airtime when recharging

I am writing this to raise my concern regarding the free airtime that you received when recharging. This information is misleading customers because it's not actually free as they say. If you switch on ur mobile data, the free airtime is prioritize before your mobile data can be used. That's the explanation I got from vodacom customer Care.Why use airtime instead of mobile data for internet connections.

I have my phone setup not to use airtime for data and yet the opposite happened.