I've seen Battle.net cards in stores around the US. The one problem with them is that there was tax on it for some reason. At least that is for the Blizzard $20 card that I have seen.
Hm, there must have been something that changed in order for there to be an official post about them today? I'll see if there are any updates to make this announcement clearer.
Sounds like a program update to me from the marketing side of things. Battle.net Gift Card sounds better than Battle.net Balance and you're more likely to share it with others, as the emphasis on "balance" was more to do with the defunct D3 RMAH. "Gift Card" definitely sounds more marketable. It's also possible this might finally replace WoW time cards that I still see sold in plenty of places while now covering HS and HOTS too.So in short, streamlining and marketing reasons, one card to rule them all