Warcraft gold, sure. RMB, I hope not.

When you charge your credit card, on the bank's books these are marked as retail credit. The bank expect this portion of their portfolio to consist of consumer expenses and manages default risks accordingly.

Banks also provide credit for currencies / investments, but those are containerized in another portion of the portfolio with different risk management practices.

Mixing these two portfolios together will make risk management tricky.

The card terms prohibit cash equivalent purchases.

RMB is cash equivalent, so no.

World of Warcraft gold is not^ cash equivalent, so yes.

^You can trade WoW gold for another in-game currency, “Battle.net wallet”, which is pinned to a 1:1 ratio with various currencies but cannot be withdrawn and thus does not qualify as “cash equivalent”.

Why should you be able to buy rmb? Card in general have specific provisions against buying cash.