Many people have mentioned this. However, I remember when the speed limit reading feature was first introduced, there were a number of posts by people pointing out situations where it had misread speed limit signs, or cases where similar systems had misread speed limit signs, or the potential for not-very-funny (fatal?) pranks where someone with a spray can or sharpie changes a "35" sign to an "85" sign. I, too, would like it if the speed limit was auto-adjusted for me, but first I'd want to know with confidence that these scenarios had been addressed. (Limiting the maximum amount the system can change the speed limit autonomously superficially seems like an okay solution, but it also limits the usefulness of the feature.)
There is also the limitation that the car currently only updates the speed limit as it passes the speed limit sign post. I, like many people, often speed up when I first sight the increased speed limit, and I almost always begin to slow down when I first sight the decreased speed limit, having had one too many bad experiences with speed traps.