> The difference is that those other are not meant to be used as a money replacement
That'd be a big surprise for me when I'll try to book my next flight with miles. I certainly thought they'd replace money.
> In Pokemon Go there are coins and they are nothing but points to spend in store.
That's literally definition of money - abstract points that you can spend in the store. Well, the definition of fiat money, to be precise, but we're not going back to gold doubloons anytime soon.
> that would be probably a legally grey area.
Transferring money to other person can certainly land you in legal gray area if it against the laws (money laundering, tax avoidance, fraud, etc.) So what's new here?
> (moreover I am not sure how you would tax such a currency)
The same way you tax any other goods. If you receive income, you are taxed on its value. You think if non-USD payments weren't taxed, nobody would have a bright idea to be paid is silver bullion? IRS is not stupid, you owe taxes on any income, and better believe it, if they don't know the exact value proving it to them will very soon become your problem, not theirs.