Going to the lowest value is incorrect if the goal is to make good use of the available dynamic range available to human observers, which is the goal around which auto-ranging algorithms are designed. If you have a white background and use a colormap primarily driven by intensity, as in your example, one end of that colormap will be similar to the background, and one end of it will be most dissimilar. Humans will naturally perceive value as contrast with the background color, and thus the highest plotted value should correspond to the color that is most dissimilar to the background. As long as you have correctly oriented your colormap so that it has contrast with your background, as you should always do (never inverting it so that the lowest-valued points are the most visible!), then when there is only a single color to auto-range and plot, it should be the highest end of the colormap (most dissimilar from the background), not lowest (most similar to the background). With only one value, the plot is then maximally using the available dynamic range to convey plot values, which is what the explicit goal of auto-ranging is.
If you instead want to convey absolute position on a fixed scale, you can certainly do that, by supplying such a fixed scale, in which case there is no auto-ranging. But if you are using auto-ranging, I argue that using the least discernable contrast in any case would be an error, not just an arbitrary choice.
Here my main argument is that the unintuitive behavior you are seeing is not to do with Datashader’s auto-ranging, but to the fact that the auto-ranging is revealing the incorrect choice of colormap. E.g. think about auto-ranging in other contexts. If your camera were auto-ranging trying to show a full-contrast image, and only two distinct light levels were found in the image, wouldn’t you expect one to go to the minimum value available and one to the maximum? That’s all that’s happening here.