-- Last Updated: Jun-21-12 7:05 PM EST --
It doesn't appear to me that you really understand the principle of why mounting the bars substantially farther apart, at least in the case of canoes with their large twisting moment in crosswinds, will reduce the stress on all mounting points by a factor of several times, or even that doing so is a desirable goal (the context of this includes your misunderstanding of end tie-down effectiveness, at least for average methods). Further, my point has nothing to do with liability, but rather what makes sense from an engineering standpoint. From that point of view, without any info from Thule, I'm trying to envision a logical explanation for mounting bars within about as little as 1/5th of the available roof-line length as they recommend for so many cars, and can't come up with one. It's not as if the roof-edge reinforcements don't run the full length of the roof, so what's the real reason? Back in the days when roofs had rain gutters, that little fold of metal had all the strength that was ever needed and then some, and people put their cross bars where they wanted to, often as far apart as roof length would allow. Modern cars have internal bracing along the roof edges which are plenty strong (this bracing is the framework for the door openings, and even to the rear of doors (like in a station wagon) it works as "truss support" for the entire car body) and Thule is not making use of what it can do, which is why I'm betting their recommendations have nothing to do with what works best for carrying boats.
I bet Slushpaddler is right. The answer is probably Lawyers. What's decided in a lawsuit can never be expected to stand up to logic, and if you ever get involved in such a thing you'll see what extraordinary lengths the plaintiff's lawyers routinely go to to eliminate every potential juror who's educational background or work experience makes it appear likely that they will understand what is going on (when the case is flimsy, the plaintiff's ideal juror is a person with no education whatsoever and a job that that requires minimal skills). By blindly enforcing a rule about rack mounts only being placed at junctions of support cross-members, Thule might hope to eliminate the opportunity for some some _____-head lawyer to win a huge settlement based on nothing but smoke and hot air. Yeah, I'm guessing again - trying to think of some reason that actually makes sense.