It was done due to badly written tools and scripts firing up instances in Availability Zone A every time.

probably to prevent folks from all stacking up in a single AZ.

Just think if someone posted a blog post saying "I've noticed that EBS performance is far better in 1d vs. 1a"

Yeah, for me, 1d experiences the lowest load of all zones. According to the pricing history for spot instances, 1d experiences the fewest price spikes compared to 1a and 1b. I'd be interested to see if other users have noticed the same thing for their zones.

I find that random(5) is the best performing. For okay but consistent performance, random(5) is decent, but you should definitely avoid random(5) due to high load.

Well, they also return errors if a zone is out of capacity. That seems like it would guard against the issue a bit. But maybe they just don't want to have to field loads of questions about that.

Supposedly it's for load-balancing purposes. (Most people spin up their machines in 1a.)

Humans are predictable - it wouldn't spread the load across zones very well.

I believe it's also a security measure.