I think people have a right to expect "find contacts in your address book" not to mean "indiscriminately spam contacts in your address book", especially from a service marketing itself as being a professional network and whose user guidelines expressly forbid you from connecting with people you do not know. Its UI explicitly violates the guidelines it expects users to follow by defaulting to inviting people -thousands at once- and not asking you to confirm the nature of your relationship, never mind giving you the chance to personalise your email.

For that reason I've purposely avoided intentionally connecting my address book to LinkedIn (other than registering with a gmail contact address) and yet all its "suggested contacts" are clearly people I once shared gmail mailing lists with. It's clearly pretty aggressive about finding contacts in address books at one end or another.

It's not like the UI behaviour is consistent - sometimes clicking "connect" on an individual's profile allows you to edit a message before sending it, sometimes it sends the autoinvite email without further prompting (which is a great way to look like a dick to someone you haven't spoken with in three years). Sometimes if you choose not to edit an English message it decides to edit it for you, sending a user you've corresponded with elsewhere exclusively in English an invitation in German. Basically, if you designed a tool to help people piss off their business contacts, you'd probably come up with LinkedIn.

And yeah, some people don't know what browser tabs do. But it's not like LinkedIn has a visible "undo" feature...