Those people normally get the wireless router up and running and forget / don´t know about the security risks involved. If the router is not thoroughly secured others might use it to surf the web itself and do things far worse than that. It´s basically a free connection to the internet (and probably your pc as well).

The informit.com article Home Network Router Security Secrets gives advice how to reduce the security risk. Its 11 chapters address each a certain security risk and give tips on how to avoid or reduce the risk. For example chapter one “Turn off UPnP” advises you to turn of the universal plug and play service.

A Trojan horse or virus on a computer inside your network could use UPnP to open a hole in your router’s firewall to let outsiders in.

If you are using a wireless router make sure you read the article, it´s worth the time.