If you are not a member of LinkedIn, then they will not be sending you emails. If you look at the email headers, you will almost certainly see that this email is not really from LinkedIn.
Good point. Spammers and scammers could easily use Linked In for their own purposes. But Linked In is definitely getting more aggressive in its own efforts to sign up members.
My husband left LinkedIn a year ago. He still receives emails from people wanting to link to him. They DO come thru LinkedIn, but they are NOT from the individuals. LinkedIn appears to generate these emails from their files. This has also happened to another friend of my husband who had quit LinkedIn.
FB did this for a time, as well. I would get invitations from actual friends and acquaintances, but they did not send them. FB did. I never responded thru FB and never joined and the messages stopped.