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.

I chose it because I wanted to learn Rust by implementing my own BTreeMap struct. I admit it’s not a good choice performance-wise, and made the comparison with Nim less meaningful.

I’ve updated the code and article with HashMap. It runs about 6~7% faster than BTreeMap.

Slightly confusingly, the authors BTreeMap just appears to be a binary tree, while the standard library BTreeMap is a B-tree: http://en.wikipedia.org/wiki/B-tree