It's a poor choice on the author's part

http://doc.rust-lang.org/std/collections/

    Use a HashMap when:
    
    - You want to associate arbitrary keys with an arbitrary value.
    - You want a cache.
    - You want a map, with no extra functionality.
    
    Use a BTreeMap when:
    
    - You're interested in what the smallest or largest key-value pair is.
    - You want to find the largest or smallest key that is smaller or larger
      than something
    - You want to be able to get all of the entries in order on-demand.
    - You want a sorted map.