I hate their usage of dark patterns to trick users into giving them access to their email contacts. I consider myself a fairly savvy user of the internet, and apparently I fell for it. (When I checked a while back, LinkedIn had access to my Gmail contacts despite my never intentionally giving them such.)
And as others mentioned, they send out emails that purposely look like some LinkedIn user has sent something to you specifically.
Sometimes they get access to your connections because they shared their inbox, intentionally or otherwise. I get suggested contacts to all sorts of people that once sent me an email.
If you hover over "Connections", select "Add Connections", and then "Manage imported contacts" in the top right corner. I had to hunt around to find it.
And as others mentioned, they send out emails that purposely look like some LinkedIn user has sent something to you specifically.
I think they're utter scumbags.