Because other players purchasing this encourages more microtransactions in a game thats already pay to play and subscription based. More time is spent making stuff for the store, that could otherwise be spent making in game earn-able content.
It's also a source of revenue for them, which to be honest i'd rather they be relying on subscriptions so when people are leaving their game they might be more keen to do something about it.
You not doing something doesn't mean there is no effect from this.