In my opinion...
While there may be legal / contractural considerations for any person sharing an unsecured WAP, from a users perspective any WAP which is 1. unsecured and 2. openly broadcasting it's SSID should be considered to be completely free and open to unrestricted use. (And it could also be argued that hiding the SSID does not constitute an attempt to secure a network).
If this is not the case, then it would in fact be illegal for anyone to access any open network anywhere without being privided with explicit permission / instructions for access by the WAP admin. Any issues relating to the charge made by the ISP for internet access and restrictions on how it can cannot be shared are entirely the responsibility of the WAP admin as the user rowsing for networs has no way of knowing if such conditions exist and what they may be!
I also feel that anything communicated within the SSID should not be considered in any way to communicate instructions / permission in relation to accessing the WAP as the SSID is not necesarily always configured by the WAP admin (e.g. router default SSIDs) and could say virtualy anything, therefore, a network named "Free Public WiFi" should not be considered to be open to free and unrestricted use based upon the SSID, but solely upon the fact that it is unsecured.
This also means that anyone running an unsecured network should be considered to be giving permission to anyone and everyone to freely connect to the network and utilise it's services such as internet access quite explicitly simply by the fact that the network is unsecured. Anyone stupid enough to leave a WAP open out of ignorance frankly deserves to have their monthly Tiscaly bandwidth allowance used up in a few hours of iPlayer viewing / media streaming by their neighbour, although if their ISP has provided them with wireless equipment without default security and/or information on how to secure the network and the importance of doing so should certainly have redress with their ISP if they become the 'victim' of such usage.
Using any form of hacking / spoofing (e.g. WEP sniffing / MAC spoofing) to access a WAP is and should remain illegal, but not under the law mentioned in the article. (Which should not even be applied to these scenarios IMO)