It's less a matter of liability and more a matter of engineering. The engineer designing/selecting the CF steer tube wants to go as light as possible without exceeding the amount of deflection (bending) the headset bearings can tolerate. To do those calculations he/she needs to know the moment arm length above the top bearing and the maximum force that will be applied. If the engineer assumes 5" of unsupported steer tube, it will be necessary to select a much heavier tube to meet the design constraints. So they pick a "reasonable" number based on typical use and design to that.

No one is saying that if you exceed that number the bike is certain to fail. They're simply saying that they haven't done the analysis, so they don't know what will happen. Maybe reliability drops by 0.001%, maybe it drops by 1%. The bottom line is that exceeding the spec is adding an unknowable amount of additional risk to your riding. Some people are ok with that, some aren't.

I figure that given the plethora of different frame geometries available (a nearly infinite number if you include full custom) why not get one with a properly sized head tube so you don't have to push components beyond what they're spec'd to do.