It's not a mainstream pro camera, nor does it sell at the price of one. 😱
But as to the why, the marketing of Fuji X cameras seems to emphasize non-studio applications (weddings, travel). Tethered studio shooting is a niche (studio) with a niche brand (Fuji) within a niche (mirrorless). To justify the cost of developing the plug-in, they have to sell it for 29 bucks. Completely rational, since that feature is not itself selling cameras to many users. The Camera Remote application, by contrast, is fantastic. And free.
Also, isn't the idea of sticking a USB 2.0(!) cable in a camera and connecting it to a computer for shooting so very 2001? If your major goal is to see shots as they come in (or show them to clients), an old iPad and an Eyefi card work very well (as long as you use the current Mobi software).
Dante