That's not how any of it works.
Not everyone uses the full amount of data.
Let's take the £10 goodybag. It offers 6GB data. However if you averaged out all the members using it you would probably find the average data use is 3GB.
I do t have the exact figures so this is a hypothetical example: giffgaff data costs them money. So at an average of 3GB giffgaff make money, if that average drops to 2GB giffgaff make more, if it goes up to 4GB then giffgaff break even, and anything above that amount would be a loss.
So now if you allow data rollover and ensure all movers use that full 6GB, the cost will now outstrip revenue giffgaff makes and hence they will lose money.