O2 Ireland, a thieving shower

I'm not the first and won't be the last to rant about O2 Ireland. Here's my experience:
Dec bought me an iPhone, most fab XmasBirthday present ever. It's prepay, so I am free from contracts and plan to unlock it to switch network.

Dec 26: Pop my mums billpay O2 sim into it to have some fun.
Dec 27: Purchase O2 prepay sim to tie me over till unlocking
Dec 28: Mum receives text from O2 - cost effective data bundle added to your bill, €7.50 p/m.
Jan 2: Mum gets same text again, shows it to me, I say "you never asked for it, get rid of it." Into O2 shop, queue for 30 mind to be told call 1909.
Jan 3: Call 1909, told to keep the add on to save on data charges as she spent €70 that week. Jaw drops.
Jan 6: Receive text from O2, on prepay Im charged 99c per day up to 50mb download allowance.

So it's 'cheaper' to use data on prepay? Why not the same cap on billpay? How are they allowed to add on extras to their customers bills without asking them first? I'm not letting this go. If anything, they should at least get the message they're robbing people blind and can't get away with it with every misfortunate customer. 

Advice on where to start with my complaint would be great btw.