As per a product manager in the UK I spoke with, the stiff damper was specifically decided upon to limit bump steer and driver fatigue in off road and rutted driving; a softer damper (all the aftermarket options, except potentially the adjustable Fox on very stiff settings) will transmit more of that jarring to the driver.
Fitting an adjustable shock to a production vehicle is very rare to find, in my opinion this is because of the added cost and liability associated (warning labels exist for a reason). An additional consideration is longevity of the part. The Fox steering stabilizer (ATS and Performance series) sold by both Owl and Agile has a life of 50k miles with exclusively on-road driving, 10k miles with 50%/50% on/off road driving, or 1,000 miles of off-road driving (specifically stated in the ATS manual, the off-road mileage is left out in the Performance manual). While it may not be a big deal to many, doing any one of the BDR routes in the US means the shock is at the end of its life and must be rebuilt from just one trip. Which, if you don't have a Fox distributor near you that can rebuild them in-house, means you're without the stabilizer for some amount of time to have it shipped out, rebuilt, and shipped back. If you aren't handy enough or willing enough to do the replacement yourself, that also means paying someone else to do it.