Given the percent of the map that falls in the 0-50 range, there's really no reason for it to be 5 (or 10) colors. Conversely, the 90-100 range accounts for a very large amount of the map, and being able to differentiate it is valuable.

This isn't like a graph where the scale changes in the middle to obfuscate the data; it's making the map more readable and providing more information, it just requires the user to actually be literate.