Free WiFi anyone? That's basically what it is since the WEP key is given away in the router's MAC address that is being broadcasted. You could almost look at it as a secret perk for FiOS customers to get free WiFi from other FiOS users.
Not just a perk for other FiOS customers... for anyone.
Beware of anyone parked outside your house... as they may be using your WiFi. »Stealing Wi-Fi Access...
I suspect most WiFi stealers are just interested in some free convenient Internet access, but they could also be doing illegal activities on your FiOS connection, or their laptops could have viruses which are attempting to infect your computers (which is easier since they are inside your firewall now).