The entire philosophy of Arch is to put user in control. The PKGBUILD format is plain-text and reviewable. The documented best practice has always been to read the PKGBUILD and the .install files before building.
I'm not saying they shouldn't look into measures to make it less prone to such attacks, but "take it down" is a very stupid take. If people can't deal with the existence of AUR, there's plenty of different distros to choose already.