AUR is an effectively unmoderated user repo. It’s not Arch Linux’s core repository, nor is it enabled by default or indeed even possible to use without manual downloads from outside the package manager.
You have confidence you are actually downloading the same foo as everyone else. And if there were an issue there would be pushback. Not so if you get some random exe from warez.com
Yes it isn't perfect, it's still a lot better than windows land.
I think you can probably find a better example, even if less recent. The AUR is unofficial and not properly vetted in the same way as the actual Arch repos, Debian repos, etc.