I think this quote of your own exemplifies what you mean very well.................
"Speaking about what is natural, a starving animal in the wild MUST move or it will perish. The same turns out to be true about fasting lab rats. Studies show that when forced to fast, they increase their physical activity by up to 50% compared to the fed rats".
A "starving" animal would indeed need to move because they are on deaths door. They would need to move as a last resort in the search for nourishment because death would be imminent; a last ditch effort to preserve life.
However, you have made no distinction here between "starving" and "fasting": anyone who knows anything about abstaining from food will know that fasting is living from ones stored food reserves (physiological), and for whatever reason that may be (a voluntary or involuntary action); whereas "starving" is when these food reserves have become exhausted, and the body begins to feed on the vital organs (pathological). The demarcation point here is the return of natural and rampant hunger (to which I can testify).
"It so happens that healthy people also tend to increase their phys. activity when fasting. Some even report that they start _craving_ movement at the onset of a fast".
I have never denied that a "healthy" individual may crave exercise while fasting if that is what the body is signalling that it needs, and I would not disagree with that at all. I have alluded to this earlier where I have said that the most important thing is to: "listen to ones body" as to its needs and wants.
Bear in mind that Shelton insisted on bed rest for his patients, because they usually came to him after already being the rounds of "Allopathy" and were in an emaciated diseased state; this is why fasting has always been known as "the rest cure".
What is appropriate and suitable in the healthy is not necessarily appropriate and suitable in the sick.
In addition: if "protein-sparing" on a fast did not exist, the body would "target" the vital organs as a source of fuel almost immediately; the heart, the lungs, the liver, kidneys and so on, and we would die in a very short time frame.
The body of course will catabolize some muscle on a water-only-fast, (I have never said that it would not) and esp' so in the early days, that is until ketosis begins in earnest and fatty-acids become the primary fuel source, where protein is "spared"
So by all means exercise while fasting when healthy if that is the need of your own body, but when emaciated and ill, the body will signal innately, that exercise is not to be undergone.
As I keep on saying, everything depends on the demands and needs and wants of the body itself, if we but just listen to it.
One more point I would like to stress, is that the body is constantly striving into wellness and good health on its own accord, but only as long as we provide it with the "right conditions" that are in proportion to the bodys physiological needs. Fasting therefore is and can be a useful tool in Natures arsenal of achieving wellness and superior health. The danger is in the "obsession" and "preoccupation" with fasting to the exclusion of all of the other elements of health.
Health is our primary goal where fasting plays a part, but only a part.
I can best summarize my position here by quoting Robert Walter MD...........
"Natural Hygiene Science "cures" sick people by removing the cause of disease. It removes the disease by removing the necessity for it. It believes Nature is right, and hence does not seek to thwart her operations. It DE CLAREs that disease is a natural process of purification, and should not be stopped, but aided. It's remedies are Natures health preservatives . Obedience to Nature is its greatest panacea. Air, Light, water, exercise, rest, sleep, etc., in such manner and degree as Nature can use are its curatives. Remove the Cause, and the effect will cease. This is Natural Hygiene Science".