Wheel balancing caused tyres to wear out more than 3mm tread in les than 2 months

I went to Tiger Wheel and Tyre Cape Gate Cape Town, to do wheel balancing and rotation in June. On their report, my tyres all had 3 mm treads. Early this month I noticed the back tyres were almost showing canvas, i.e. 0 mm tread; these were the tyres that were in front for almost a year [and had 3mm 2 months ago]. The front ones [the ones swapped from the back, now still have tread around 2 mm]. So, we concluded that, there is something they did during wheel balancing that is causing the tyres to wear out that fast [more than 3 mm worn out in 2 months, yet the same tread on [now] front tyres, is around 2 mm]. I am no expert but, the same tyre took a year to wear out 1 mm, but after wheel balancing, it wore out more than 3 mm because the canvas is now showing on both back wheels. They tried to dodge by say, the tyres are soft rubber; yes, but the ones that were at the back, they were not worn out at all from last year we fitted them

Now the question is, I went to the service so i can have more mileage on my tyres, but now i have to buy new tyres which I have not budgeted for, now the question is who is supposed to pay for those tyres [i.e. more than 10000 rands for 2 tyres; or who is supposed to pay for all the traffic fines every time I am pulled over by the traffic officers.

Maybe if you own a Q7 [or an Audi for that matter], better not do wheel balancing at Tiger Wheel and Tyre, Cape Gate, because their machine makes your wheels EAT the tread so fast, that in a month or so, you will need to buy new tyres