Okay, at the risk of sounding arrogant, you have no rights to complain if you give out permission to 50 different websites without knowing what they do. You cannot complain that you are being exploited, when you allow yourself to be exploited. My google account is linked to fifteen different sites which I know what they are all for.
> go check your inbox for an “Invitation to connect on LinkedIn”. They don’t come from @linkedin.com, which THEY ABSOLUTELY SHOULD
Again, you gave them the permission to do so. And to play the devil's advocate, would you be less* likely to accept an invitation from your friend if he sent you an e-mail from some foreign account, or his account of which you are familiar? Again, you gave them the permission. If you don't like it, just make a linkedIn account without a connection to Google account. You've not done so because connecting to Google is convenient. You are paying a fair price.
I get minimal number of e-mails from linkedIn because I didn't give them permission to send me every gritty details of other people. It does exactly what I told it to. Maybe you should as well.