Security vs usability
If you go for a walk or a drive through central Stockholm with a WiFi device in your pocket, you'll discover many wireless networks called "NETGEAR", "linkys", "Apple Network" or other default names, and no security turned on. Not having to worry about passwords is convenient when you set up you home network, it makes the system more usable, but it's obviously not secure.
To an architect, security and usability are examples of quality attributes, non-functional aspects that need to be considered when designing a system. You usually can't have everything, trade-offs have to be made.
In the case of home networking, the architect seems to have decided that usability is more important than security. But who would this architect be?
The author(s) of IEEE 802.11b?
The architect of the access point?
The architect of the connecting device, usually from a different vendor than the access point?

None of the above have architected the actual system you are using, since the system of interest consists of both access point and device. The authors of the standard could have mandated a solution, but didn't as far as I know. So the result we're seeing is a kind of emergent behavior.
Emergent behavior is a powerful concept, but in most systems you want more control over the result.
So if you want to avoid surprising emergent behaviors in the systems you develop, make sure each system (and a system can consist of systems which consist of systems, etc) has an architect who makes conscious trade-offs between quality attributes based on collected requirements.