Until now I had assumed that stealing an unencrypted wifi connection was just bad form, not actually illegal.

But see this news article about a UK Court cracking down on a dishonest Wi-Fi user

In one of the country’s strictest ‘Wi-Fi’ sentences, a man was found guilty and fined of £500 by the British court of dishonestly using a wireless broadband connection used by households without any permission.

Judges at the Isleworth, Middlesex court convicted Gregory Straszkiewicz, aged 24 for acquiring a communications electrical service as well as for owning a device that was deceptively utilising a communications service.