You can disable it in your Chrome settings: chrome://settings/content/federatedIdentityApi

Websites that choose to put a sign-in with Google button on a page can disable the popup by setting data-auto_prompt="false". The default being "true" is how Google is pushing this, but this seems like a rather gentle way of pushing.

It's clearly a deliberate choice that websites make. Your explanation as to why they're doing it seems very plausible to me.