The law? Which law?

Not to be any more of a jerk than necessary, but such statements really ought to be accompanied by at least a mention of what nation, preferably of what court (or whatever) has regarded it so, and ideally a citation of the precedent/decision under discussion.

In the US, I'm not aware of any cases involving a home network deliberately left open. The most likely issue would be a DCMA case against the network owner.