What does transparent mean here? AWS is super clear what resources are zonal and provides tons of guidance around making things multi-AZ. AZ outages aren't exactly frequent but they're reasonably likely.

Being susceptible to AZ (or region) outages is very much an architectural decision. Or a bug that needs to be fixed (I'm sure Coinbase didn't YOLO single-AZ, they've undoubtedly learned about some edge case that needs to be fixed). Sure it may not be worth the cost/complexity for some systems but resiliency is like job one for anything in the cloud that costs money when it's down.