> If you take your car to the shop and can't pay for the work to be done, they put a mechanic's lien on it, and impound the car until the work is paid for. This is no different from that method, which is totally legal.

The former is legal because it is specifically authorized by law. For example, here in Washington mechanic's liens are specifically authorized and regulated in the Revised Code of Washington chapter 60.04.

Unless website impounding is explicitly authorized and regulated in law, then you can't accurately say it is no different than a mechanic's lien.