There are a few reasons why they do this I reckon:
1) Not everyone will claim it because it's a hassle
2) They don't pay £70 for loading the £70 worth of credit
3) There is potentially some form of tax benefit / avoidance going on applying credit in this way
4) Could be some form of kickback / other incentive from the company(ies) involved
£70 credit is surely more useful than the card as you could call them up and ask for it back via BACS.