the reason they're doing this? i'll tell ya. debt. that's why.
Lets say you want to buy a new gadget, costs about 250 (currency irrelevant, lets assume it's all the same). So you can't use your debt card, and like hell you're going to carry around 250 in cash, and chances are, the store doesn't accept personal checks. What do? oh, it just so happens you have a credit card without that limit! now that's convenient, just pay later too! So the bill shows up later. Now what? pay it off, and increase your credit score, great! a wild bad habit appears! Bad habit uses massive bills! Player fainted! It's super effective! (because you always fainted then it pointed out the obvious.. damn game...)
Anyways, the habit may form and massive bills pileup, then you need to go in debt. Making the credit company very happy. the more you owe, the more interest they can tack on you, and that's how the banks make money. that's why they're doing it.
then again, i may be wrong and i think i'm missing a few details...