They acknowledge that piping to bash is controversial in their install guide and they provide other options for installation. I think they were intending for it to be as accessible as possible to non-technical users and piping to bash was the easiest way to make installation a one-line command that requires zero additional knowledge and still works on the tiny raspberry pi zero w. I can't say I agree with it as a general practice but it wasn't enough to turn me off since their software takes like 15 minutes to set up, provides a nice monitoring dashboard, and runs on the raspberry pi I'd relegated to my junk drawer. Your route may be saner to you but it certainly isn't for a lot of people who tinker with raspberry pi and want something like pi-hole but don't have extensive technical knowledge (I am not one of those people I am just a lazy engineer so it works for me too).

https://docs.pi-hole.net/main/basic-install/