@aral None of that software is from Linux or cares about what kernel you use - all they care about is the /etc/passwd, /etc/shadow, /etc/group and /etc/group- files existing and being in the correct format.

useradd is from shadow; `git clone https://github.com/shadow-maint/shadow` and requires passing flags to add a user in the way you would expect.

adduser is from Debian and is a perl script that prompts what password and groups etc you want to enter and then passes that to useradd without you having to look up the flags.

There is also superadduser from slackware that is pretty much adduser except written in GNU bash instead of perl.

Most LiGNUx distros just ask user the username and password they want in the GUI installer and then most users don't need to add another user - in case they do, most of them have a GUI tool in the settings that will add the user.

Having multiple ways to do things is fine as some people prefer adding users in a different way to other people, plus that's also very beneficial if something breaks on the system as there will always be an alternative way to fix things - for example if perl breaks on a system and you need to add a user as part of fixing it - adduser will not function, but useradd and superadduser will - although you could also just hand-modify /etc/passwd and /etc/shadow in the Church of Emacs.

Demanding that there is only one way to do things is very systemd/Linux-esque.