There probably isn't a completely accurate one, but the relevant point is the morality of deliberately and knowingly taking or using something that belongs to someone else, while assuming you have the right to do this if the owner has not adequately protected it (or perhaps doesn't know how to).
I suggest that the morality is exactly the same regardless of what it is that you take, or how easy it is to take it, because it is the fact that it belongs to someone else that makes it wrong to take it. Yet if examples are quoted involving familiar things like houses or cars, you suggest that these are not valid examples, implying (unless I misunderstand you) that you think theft is wrong for an unsecured house or car, but fair game for an unsecured wireless network. Personally I don't see the difference, because no matter how careless the owner of something has been in failing to protect it, if it doesn't belong to you, it doesn't belong to you.
The situation is slightly altered if you do it unintentionally, as, for example, inadvertently finding yourself on unsignposted private land during the course of a country walk (or is that a bad analogy too?), but apologies and explanations are usually sufficient to resolve such incidents, and decent folk would take care not to do it again. The owner of the land would, of course, be sensible to fence or signpost it, and in reality must expect a fair number of unintentional tresspassers if he doesn't, but failing to do so doesn't make the land any less his, and doesn't deprive him of the right to ask anyone on it to leave.
Rod.