Manjaro is a great distributions - underpowered - but still a great distribution.
AUR is unsupported on all Arch Based distributions.
I suggest you disable AUR in your Pamac preferences as this will effectively prevent building from AUR while using Pamac GUI.
Octopi has similar functionality.
You will still be able to build alien packages using command line tools like makepkg, yay, paru, pamac and many other.
AUR is not a selling point - it is unsupported.
Yes Manjaro provides the tools to build from AUR - that does not implicate official support of the resulting package(s).
That is correct - antivirus is wetting yourself on cold winterday - it warms for a few seconds then it gets worse.
final thoughts
Manjaro is about simplicity and freedom - so you have the freedom to build custom packages using scripts from AUR - @Ben’s comment is spot on - freedom
For years we have actively worked to educate users about AUR, the use and risks involved.