That you should be very careful about what you install. Cut&pasting some line from a website is the exact opposite of it. This is mostly about psychology and not technology. But there are also other issues with this, e.g. many independent failure points at different levels, no transparency, no audit chain, etc.
The counter model we tried to teach people in the past is that people select a linux distribution, independently verify fingerprints of the installation media, and then only install packages from the curated a list of packages. A lot of effort went into making this safe and close the remaining issues.
Be careful who you trust when installing software is a fine thing to teach. But that doesn't mean the only people you can trust are Linux distro packagers.
I think it has a lot to do with "curl|bash". Cut&paste a curl|bash command-line disables all inherent mechanisms and stumbling blocks that would ensure properly ensuring trust. It was basically invented to make it easy to install software by circumventing all protection a Linux distribution would traditionally provide. It also eliminates all possibility for independent verification about what was installed or done on the machine.
Downloading a deb via a package manager is more secure. Downloading a deb, comparing the hash (or at least noting down the hash) would also already be more secure.
But yes, that the run arbitrary scripts is also a known issue, but this is not the main point as most code you download will be run at some point (and ideally this needs sandboxing of applications to fix).
> Downloading a deb via a package manager is more secure.
Not what I meant. Getting software into 5 different distros and waiting years for it to be available to users is not really viable for most software authors.