The answer to the last question is that there is a fair bit of old school carrier grade telco equipment that uses external PSUs and whatnot. Aka, what Verizon installs for extending 2G/3G/4G (2G is still used for 911) in places where they have poor tower coverage.

However, we're in agreement. There seems to be little reason to have a case which cannot accommodate the PSU to be an internal component and allow for proper rail mounting and max density. If anything, I'd put this in the category of making things more difficult for KNC to sell to massive operations - it's annoying, but not necessarily a deal breaker. Maybe there's some other reason that I can't think of why they'd design the case as such - beats me though.