Its not good taking people's money/credit without permission! Even you turn off your data on android, it seems like there are always a chance for Digi to take your RM8.00. Best part is you will receive SMS telling you that you have used you internet to maximum you can use free internet until midnight.
I believe it's a very known issue for all the smartphone users regardless what brand and platform they are running with. There will always a chance for the device it self to connect to internet which i dont know how. It is happen to my prepaid as well and what i did was i called to cs to deactivate data service until i get my own internet package.
Btw ive been having same issue when i used telco C and M as well.