At the time, Linked In wouldn't even let you opt out of mail without making an account, which is definitely never happening. (Thankfully, they eventually added a way to tell them to not contact me again, but I'm going to carry my grudge)
At some point I spent some hours to investigate how they did it and trying to tell google (gmail) to not let Linkedin do it (again?). Not sure how much success I had. I think it is a mistake/problem of Google to allow such things.
I don't really understand how you make the jump to this part
With the abusive relationship example, if someone's gone as far as snoop through your phone and text your friends without your knowledge, there is no telling what they'll do next. It's common for someone who does this to also be a verbal and physical abuser, which is rarely worth sticking it out for, regardless of how well-connected they are.
With the specific LinkedIn example, they may e.g. sell your data to third parties, leak your data publicly via irresponsible data storage, and allow employees to access your data without authorization. Not to mention spamming your entire contact list with LinkedIn invitations in your name.
Is that worth it to you in exchange for some networking?
If you use it right, it can be useful. LinkedIn isn't your friend, but you can use their platform for your advantage.
Act 1: When I'm not paying attention, my friend says, "hey, do you mind if I go through your address book and contact all your friends and family and ask them to be my friend too? If you consent, say, 'what' or 'huh'"
Act 2: When I'm not looking, this friend goes through my address book and spams all my contacts with whatever they're selling under my name.
What would you advise me to do after that? You'd probably say, this person is not your friend by most definitions of the word, you should cut them out of your life and ask them to not contact you anymore.
To which I may say, but this friend is well connected, they might be able to get me a job in the future. (Although I've never gotten a job or even a lead through LinkedIn personally.)
What would you say to that? Probably something like, they're abusive and deceitful, whatever they have to offer, they'll probably damage your life more than they'd help you.
And that's why I closed my account and added a "delete on sight" filter for their emails.