You misunderstand my point. R6 "currency" is not the renown you earn in game. It's a currency conversion. In this case, I pay you actual money, and in return you give me back monopoly money. With the understanding that monopoly money can be used to buy in-game cosmetics or boosters. Now setting aside whether or not agreeing to these terms makes me a sucker, there is the issue of these terms being subverted by the publisher not allowing their monopoly money to purchase their in-game items. That is fucked up.
In the case of videogames, and specifically this example, it's probably just a half-assed oversight. It's kind of silly, really.
But in principle, for example if you substituted videogames for something more meaningful, then I could imagine the practice might even be against some laws. It's predatory in nature. Before you laugh, understand that I'm not asserting that is true in this case, of Rainbow credits (I've already said it was silly), but I will go on record to say Ubi is showing poor form here. And it does not inspire confidence in their "games as services" economy.
So imo a gross oversight really. But they should be called on it, because this is the direction Ubi and other publishers are going.