The freedom to take a device and modify it to run your own changes is what the GPL is all about. Let's not forget the modern open source movement was the result of someone not being able to load a custom font into a printer. Do you think Stallman would've been satisfied if Xerox had given him the source code he asked for, but then indicated they'd sue him if he tried to load any modified software onto the printer?
AVM's interpretation of the license ("you can modify the GPL software but you're not allowed to flash it onto the device") was shot down by the court, and rightly so.
I think a company actively going after another to prevent exercising the software freedoms granted by the GPL makes them deserving of being called "notorious". It's a shitty practice in general, but it's especially shitty when you do it for GPL firmware.
