Well, I don't know much about routers, but the popular software firewalls, like ZoneAlarm, Norton, McCafee etc.. block internet access by PROGRAMS, and not ports (sometimes ports also but I'm sure that nobody uses it), therefore, if a user set the firewall to allow LimeWire or any other P2P application server right, that user will have no problems uploading, because LimeWire WILL have access to it's ports, while others programs won't.
I am using a firewall, and I have no problem downloading\uploading.
Maybe people don't configure their firewalls properly, but having a well-configured firewall is good.