Modern wireless networking products are inexpensive, simple to set up and very convenient. They are also full of holes... security holes, that is.
The reason for this can be traced to the popularity of wireless. Wireless networking is now firmly entrenched in the home computer market, and that means that the people buying the products cannot be assumed to have any form of expertise in IT; hence, the products had better be extremely easy to set up, and use.
Ease of use and security tend to be mutually exclusive. So for wireless manufacturers, the reasoning goes that while the average Joe or Jane Smith at home has nothing they'd really care to hide from the world, or that the world would really care to find out about them, they will get very peeved if they cannot get their new wireless router up and running quickly and without help.
Unfortunately, the only way to make a complicated appliance like a wireless router idiot proof is to use default settings that will not require the user to change a single settings on any of their other wireless hardware. By extension, this means that any wireless device can connect to said router using default settings.
To repeat, this means that ANY wireless device in range can connect to the Smith's new router. Now, if you are Joe Smith, do you have anything you'd like to hide?