Theft of data

Once again i ask Vodacom to answer the below question...

Why if i have a whatsapp 30day data bundle, and i buy normal day to day data, does my day to day data get used when i am using whatsapp???

Stop wasting my day to day data on whatsapp use... how simple can that be.

Because now what happens is that i buy my day to day data to use for other activity and vodacom keeps eating it when i am on whatsapp, therefore i have to keep buying more data,..then when my day to day data is diminished they use my whatsapp ticket...how ridiculously ridiculous is that?.. how many people are you doing this to?