1. I think McDonald's makes you make an account these days to use their WiFi, I could be wrong, however. This means you now have identifying information about you and your connection on McDonald's servers, which can be subpoenaed in an investigation.

2. Local libraries generally require you to have a Libary Card, which is associated with your library login. Because lots of people need library computers, they need to be able to get people to leave the computer without having to physically remove them. One of those ways is with tracking who is connecting by associating their account with a library card and disconnecting them automatically after a period of time. Your library usually wants you to live (or at least exist in) the neighborhood, and will have your name and address on file. This can also be subpoenaed in an investigation.

3. MAC address spoofing is trivial, sure, but a lot of folks don't know to do it, and in any one of these cases, if they used the MAC address that was burned in when the product was sold, they have a real opportunity to nail down which specific device connected. They would subpoena the manufacturer for records of where that device was sold, they would subpoena the records of the local retail outlet, and have information on the sale of the device used to connect.

4. In your neighbors case, you could just be condemning them to civil or criminal liabilities, depending on the laws in your jurisdiction. That's a pretty "not nice" thing to do.

I'm sure there's plenty of others. Those are just off the top of my head. Most of the reasons involve "companies keeping information about who uses their services and how," often called "logs."

There's a lot more information than just an IP being sent in a TCP packet.

To be clear, I'm not the person you were responding to originally and I think OP is fine if piracy is legal where they live, you just asked how it would be possible and so I tried to think of ways related to the suggestions you made. Cheers.