Currently the list of scales is hard-coded, which does help us ensure a user does not enter a custom scale factor incorrectly.
But we do have plans to pull the scale list into the Detail Preferences, to enable editing there.
In the meantime, where possible we get the scales added. In this case, those are clearly overall "key map" style details, and at the bottom of the scale list will not get in the way. I will get this added to the wishlist.