This is because you are using SSO (Single-Sign-On) on a web page by clicking somewhere on a Google icon to sign on with that, versus signing on direct with a user-name/password in a non-web direct-access sort of way. That whole information page you took a screen shot of is standard disclaimer for that type of situation. The good news is that you don't have to use SSO if you are truly that concerned.
In my opinion it's fine, but if you are worried then use System Preferences > Internet Accounts to add a "Google" account, and only enable Calendars and not email (or notes, contacts etc.) for that account and service. You can then use the Calendar app on your Mac and it will have access to just your Calendars.
SSO is very normal these days and you seeing that screen is more about modern disclosure and privacy laws, versus how it would traditionally be buried in an end-user license agreement. So in general I honestly wouldn't be worried, they are just being in your face about it because the laws require that these days.