The scale intervals are directly tied to the basemap in the web map. If you are using ArcGIS Online default basemaps, then you can't change the scale intervals because the basemaps are tiled, meaning they are pre-created images for a given scale. They are not dynamically generated. If you have your own basemap, based on an ArcGIS Server service that is dynamic, then you won't see the scale intervals at all and will just have the + and - options. Alternatively if you have your own tiled map service, the intervals will be base on your tiling scheme.
Hope this helps.
Mike
