It's a feature that adds value. They are capturing that value via an in-app payment. If the value you would receive from this feature is less than the asking price, don't pay for it. Pretty simple, really.
One could argue that all features add value. Should all features then be à la carte via in-app purchases? That'd be absurd.
Things can be both absurd and simple. Really.
It’s price differentiation. Pure and simple. I think that’s a very reasonable way to make money.