Indeed, redemo.py does include this feature. But it is a completely different application with a very specific goal - testing regular expressions.

On the other hand, IDLE's search is meant to be used to find pieces of code in the editor, which is a significantly different use case.

Since SearchBar always sets the pointer to the first match, as well as highlighting it, there would no real point to only mark the first match.

As for disabling the marking of all other matches (besides the first), what is the point? Does marking all other hits really have any downsides? To me it seems like a useful feature that users will easily grow accustomed to.