First, I'd like to address this from the angle of asking about "Online Game Service/Store payment processes" in general, and not the specifics of asking about 'currency' per se, although that is definitely covered under this topic.
I can see an argument being made both ways:
Online Game services like Xbox Live, Steam, Nintendo Play Store etc are built primarily for gamers to utilise (and for publishers, but lets not open that door today).
But these services are more than just a storefront. Digital content delivery, Account/Online multiplayer management, social features (chat, friends, messaging etc) and more.
As such, they are generally known as Services, with the Store just being another asset of the overall service offering. Gamer services of course falling under the topic area:
Game-specific hardware and utilities
It's pretty close to incidental Knowledge. It's something we'd probably know by association (or be able to check in the Terms & Conditions, Instructions, or even know from personal experience.
Also: 99 times out of 100, a question like this will be easily researched or found easily. If so, they will probably attract negative attention as the OP wouldn't have done any 'homework', or even a Google search.
It's fairly close to shopping recommendation. It's hard to distinguish between 'How can I buy this' and 'Where can I buy this'
(If you have another reason against I'll be happy to add it here and address it)
I don't think we'll be getting enough of these questions to have the argument over them every time. The Bitcoin one is the exception, rather than the rule, because it is something that isn't immediately clear from the XBL payment system. I can't imagine many other questions about payment of services will come up.
I also think that if we are to cover game services & utilities in general, why would asking about payment options be any different to say, asking about using headsets to talk to friends, removing games from your account, or even participating in the Steam Summer Sale?
I think we should allow these questions. This does not mean suddenly every payment question should be left open: we should still treat every question on it's individual merits. For example, if a question about payment for online services is:
- A genuine problem, issue or topic that needs clarification: Answer (and potentially upvote)
- Easily researched: Downvote and answer
- Unclear or not useful: Downvote, Potentially Close as 'Unclear' or a custom reason why you think it's not useful
- Asking where to buy something: Close -> Off-topic -> Recommendation reason
- Listing where to buy something: Flag as Spam