If you made your house into a pub-lookalike...
with an illuminated "Tennents" sign on the wall, etc.. Then you would have no legal grounds to arrest any members of the public you find on the premises, as it it is not necessary to seek permission to enter a PUBLIC house. On the contrary, it woud be your fault for sending-out a misleading message to passers-by.
Likewise, a wireless connection which electronicaly advertises itself as being a public facility must surely fall into the same category. The fact that the owner is stupid enough to leave it like this is neither here nor there.
I'm very concerned that these precedents could lead to molestation and harrassment of persons using laptops in cars for perfectly legitimate purposes. It's another case of protecting the stupid against their own stupidity, at the expense of eroding the rights of the public to do what is perfectly legal in a public place.