An unlocked car (which is the closest legal analogy, I think) is a bad fit here. If we're really going to search for analogues let's pick one that really matches what's going on in digital communication: physical communication.

  Whoever, without authority, opens, or destroys any mail
  or package of newspapers not directed to him, shall be
  fined under this title or imprisoned not more than one
  year, or both. -- Title 18 § 1703(b)

What if the "computer" on the internet is my pacemaker, and it uploads a data stream to servers in, say, Dublin. Does that mean I should not have a reasonable expectation in the U.S. for privacy of that data? That kind of rationale is bad in principle and bad law.

If you're uploading information to a third party, you no longer have privacy rights under the third party doctrine. That's very well established precedent.

As I said in another comment here, the issue is when it goes beyond such information like IP addresses and goes to actually hacking, which involves running code on the victim's computer.