You're conflating two things, copyright and trademark.
The source should be open, that is why the open it. So then if you want to create a MyCoolEmbeddedBoard product, you can! No problem. They're not complaining about that.
What they're complaining about is not people using their designs, but advertising it with Arduino trade name. That is reasonable. Just like, if I create some software I should make my own name, and not try to tell people is the Free Software Foundation Whatthewhat. They have a right to their own name, even after they've given away the source. Indeed, having the correct name is part of knowing where your source is from.
What they want is that when people buy a piece of Open Source Hardware that has the Arduino trademark, it is drop-in compatible and the name is properly licensed. And if you want to copy their license and not their name, then you don't need a trademark license, and you can be compatible or not. You can, for example, say on the box that it is compatible with their brand, and that you're not licensed or affiliated, as long as it is clear that you're not official. That is fair use. But you can't put their logo on your product to advertise that compatibility, or use their name in a way that makes it sound like you're licensed.
This should be obvious to anybody that has ever written software. If you have, and it isn't, please learn this shit finally. That includes anybody who would buy this stuff and call themselves a "maker."