No, it's not. AUR is not Arch, and is not "supported" by Arch.
It's a fundamental problem with the security model running code from randos on the internet. If someone published a git repo on GitHub that installed malware when you ran
git clone git://github.com/user/repo . && ./configure && make && sudo make install
you wouldn't be saying that "this is a fundamental problem with the security model of git."From the AUR homepage, in big text:
> AUR packages are user produced content. Any use of the provided files is at your own risk.