Vodacom thievery

I rarely use data from Vodacom on my phone, as I have a data SIM card as SIM 2 in my phone, and use that for data, and the Vodacom SIM only for calls. I buy airtime maximum 2/3 times per month, and on the 16th of this I did my top-up, only to be told yesterday that I'm out of airtime, so I topped up. This morning I again sat without airtime, so I went into VodaPay to see what caused this airtime consumption, but it only allows you to check your data usage history. After a long and problematic session with Vodacom's Tobi, I finally got a airtime usage report from them. I was charged over a R1000 for out of bundle data, so they just used my airtime money to pay for that. I regard this as sheer *******, since they can cut you off mid call if you run out of airtime, but merrily allows you to consume data that you did not order. Should there not be a law against this form of thievery?