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.

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