Non-system software, which is what often gets installed with this method, typically does not get root privileges on my systems, or at least is not expected to write anything into directories like /usr.

These scripts are often written by people who only know one OS well (if any), and if that OS is macOS, and you're on Linux (or FreeBSD, or whatever), you can expect them to do weird shit like sticking binaries into /usr/bin in circumvention of the package manager, or adding their own package repositories without asking you (and often not whitelisting just their packages, which allows them to e.g. replace glibc on your system without you noticing), etc.

It's not comparable to simply using the already installed software.