On the other hand, microtransactions are horrendously expensive due to fixed per-transaction fees (at least using credit and debit cards), so some business models are simply not viable without stored value cards or accounts.
It's totally viable and trivially possible to just payout earned interest to these accounts. You can still issue giftcards, it's just no longer passively profitable to do so.
It's probably nontrivial anywhere interest income is taxed, but even more so in the US with its non-flat interest tax scheme (it's taxed as ordinary income at the federal level, with an income-dependent rate).