Hm, that's an interesting choice. I also wonder why they didn't use an encoding that has an alphabet with 2^n characters (that way you can directly map 1 charater to n bits when decoding).
Coincidentally, I've just finished some work on a project[1] that is in the same space (identifiers for images).
For the reasons you pointed out, I found Douglas Crockford's base32[2] encoding to be a good fit.