An admin can write to /Applications without privilege escalation? That's a macOS bug. If the operating system didn't rely on an 80s-style put-all-the-executables-in-one-place app launch paradigm, maybe there'd be less incentive for app developers to ignore the per-user Applications folder that macOS supports.
An app can spoof or abuse privilege escalation dialogs? That's because macOS doesn't implement an Orange Book-style Trusted Path. It's why Windows and similar operating systems have secure attention keys in the first place.
So yeah, Zoom is (ab)using flaws in macOS to get itself installed with minimum fuss, but it isn't doing it with evil intent. They fixed past issues; they'll probably fix this. Meanwhile, these long-standing macOS security flaws won't be addressed by Apple, who has a terrible track record about these things except when it lets people bypass their App Store.
P.S. As an enterprise customer, I'm much more worried about end-to-end encryption in Zoom, and the apparent lack thereof. I'm also not sure how that compares with other video conferencing services.