[zbentley]

It was for spreading load out. If someone was managing resources in a bunch of accounts and always defaulted to, say, 1b, AWS randomized what AZs corresponded to what datacenter segments to avoid hot spots.

The canonical AZ naming was provided because, I bet, they realized that the users who needed canonical AZ identifiers were rarely the same users that were causing hot spots via always picking the same AZ.

[Twirrim]

Almost everyone went with 1a, every time. It causes significant issues for all sorts of reasons, especially considering the latency target for network connections between data centres in an AD