The GPL is the reason it's easy to replace the firmware on a consumer router. If the GNU userspace tools were BSD licensed then DDWRT, OpenWRT and Tomato would not exist. It's actually one of the best examples of the GPL's power to allow people to do use their devices to their fullest potential.
I don't see the connection. The Linux kernel being under GPL means manufacturers had to release source code for their devices. Where does the userspace come in?
Do OpenWRT and the like even use GNU userspace tools? I would assume they use Busybox.
Well, BusyBox also happens to be GPL. ;P (Also, it should be noted that GPLv3 requires the user not only to have the source code for the software on the device, but the ability to change the software running on the actual shipped device.)
They use the GNU C Library. After a bit of Googling I'm not entirely sure what forced Linksys to open source the code, but it was definitely more than just Linux.
On a consumer linksys router for example, this is a very simple process.