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.

Do users also have to pay extra to get audio through the headphone jack?

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.

You can probably use the iPad's builtin hdmi out feature for free, but it will output an iPad-shaped display. The one you pay for will dynamically resize to fit the screen it's attached to and display different stuff. Not a huge difference but it isn't like they did NOTHING for the feature.

Why is it absurd?

It’s price differentiation. Pure and simple. I think that’s a very reasonable way to make money.

I guess they should offer a version with banner ads all over your dashboard.

Very Dilbert.