The justification is that the risk of someone using your network for illegal means is very low, while the risk of you getting hacked at the local coffee shop is potentially higher. Hence, worry about your machine, not your home connection.
I say BLAH! This piss poor argument ignores two significant points:
1) There is little or no benefit to you from opening your network; and
2) It takes minimal effort to secure your network with a password.
The risks may be low, but meanwhile you have nothing to gain. Meanwhile, the effort necessary to provide that little extra layer of protection likely outweighs the cost of that single long tail incident – one that could potential cause you tons of legal hassles.
If you are hell bent on providing web access to home visitors, I’ll take for granted that you trust them. Give them the key, like I do. Or if you’re wearing a tinfoil hat as you hand them their coffee, ask them to allow you to type it in yourself.
UPDATE: Being open can cause hassles (unless you don’t consider having your computer confiscated by less than technology savvy law enforcement officers a hassle).