Perhaps it's because I'm not yet an owner that I don't see this as some of you. I do see it as a bit of an overreach on the part of AP, but maybe my interpretation is off base. Again, I see it as a safety measure: when one activates AP, there is an assumption that the driver believes that AP can handle the current conditions, right? I mean, a responsible Tesla owner would never activate AP in conditions outside the scope of AP.
So, we all know AP has limitations, and when it reaches them, it advises the driver to take over. Everyone accepts that. But in this case, the OP showed that when pushed past one limitation (miles per hour), AP disengages and refuses to allow itself to be re-engaged until the car is re-started. There could very well be other thresholds that might cause the same behavior; I expect there are, things like high lateral g forces, or rapid, multiple steering inputs in opposite directions (like swerving to miss an object in the road too small for AP to detect.
I propose that it is actually safer in the long run (that is, until a better AP system is released) for the system to recognize when there may be factors that it's sensor suite cannot account for on a particular drive, and for it to "lock out" until the drive ends, even though it may be annoying to most. The OP clearly perceived a situation that needed to be addressed by a brief bit of acceleration, and only a very advanced AI (and accompanying sensor suite) would have been able to perceive and make the same decision in that situation. Hopefully we get there sooner rather than later. But until then, I'm not sure disabling AP's ability to take itself out of a game it recognizes it can't play is the better idea.