So basically if you hit an arrow key which will not make the cursor move within the search string it aborts.
IMHO this is a mode, eg the meaning of the arrow key will change whether the cursor is in the search string or at an end. Moreover the user focus is on the occurences hilighted in the file, rather than on the field at the bottom of the window. Since that occurence is selected, it seems coherent that the arrow keys would do what they usually do: exit the selection. Also that behavior will be familiar to Emacs users.
Fixing using backspace shouldn't be so much of a hassle since search strings are typically short, and as the results are incrementally displayed, the user can detect errors early.
-- D