Until somebody uses your open wireless for child porn and the cops come asking you questions.
In Germany this defense wouldn't really help you much. You're (partially) responsible for the crimes that are committed over your unsecured network. It's called "Mitstörerhaftung".
That kind of changes the math a bit. I don't want a dozen people torrenting off my network, not because I'm afraid of getting in trouble, but because it degrades my ability to use my network.
Is that legally enforceable? They have a search warrant, not a warrant for your arrest. How are they in a position to demand you leave the premises?
That's some scary stuff either way, thanks for sharing.