1. You download an installer.
2. You activate that installer.
3. Your system tells you that the installer can run some code.
4. You agree to the installer running some code.
5. The installer installs some software, via the code that you said the installer could run.
Seems perfectly acceptable to me. If you literally cannot open up the archive file to inspect its contents without running some code inside it, that's not Zoom's problem. It's just a really stupid decision on the part of Apple.
The message reads "this package will run a program _to determine if the software can be installed_". Old iWork updates used this to see you had a previous version of iWork on your computer. nVidia's Web Drivers used it to check if your Mac had an nVidia card. No other macOS pkg that I'm aware of makes actual changes made to your system during this step. That's not what the user agreed to.
Behind the consent message is a grayed-out "next" button that you never get to click.
I suppose this system was ripe for abuse, but that doesn't excuse the people who abuse it.
Pre-install scripts are only supposed to do things like check if you have prerequisites installed, eg if your app requires some version of Python or whatever.
Instead they are abusing that to just install everything immediately.
There is literally no reason for it beyond saving one or two clicks.
...I was about to write a reply saying "well in that case I don't understand what the GP is complaining about", but then I opened up the installer again.
I didn't fully take in what the parent was saying. Zoom is completely short-circuiting the normal macOS package install flow. You click continue once to acknowledge that "this package will run a program to determine if the software can be installed", and then Zoom is suddenly installed and the installer exits.
I can understand why they did it, but it's not good.
Although, Apple deserves some blame here. Firstly because their non-app-store software install flow has been screwed up since the release of Lion, and secondly because why can't you use Installer's "Show Files" option before the preinstall step?