There you go, here is the reason for the "awesome Blizzard Battle Coin".Research has shown that putting even one intermediate currency between the consumer and real money, such as a “game gem” (premium currency), makes the consumer much less adept at assessing the value of the transaction.
There are no "security problems" when using credit that relates 1:1 to the customers real currency (wtf would make you think that in the first place?!). The only "problem" is that people are less likely to buy something for 3$ than for 750 Battle Coins even if both actually mean the same cost.
Same reason why they put the shop directly into the game, the convenience of not having to leave the game is negligible, the point is to get people to spend their credit impulsively, having to alt-tab into a browser 1.) gives them time to realize what a freaking stupid thing they're about to do 2.) disconnects the purchase from the game, but the game->fun so with ingame shop purchases are associated with the game, therefore with fun, thus people enjoy spending extra $$$.