You can make and sell any product you want. But, you can't legally say, "Made for....(insert brand)" unless you license the name. This is why you don't see Goodyear saying "These tires are made for the Ford Mustang." They say "Sports car tires" instead. Apple had all kinds of issues with this with their iPod. Lots of products came out that said "For iPod." Apple made them pay for that right. That's why you now see many goods say "Made for portable audio" because that is free.
I fail to see your logic in the Sennheiser Legal taking issue Uncle Erik - you cannot stop someone making tyres for cars, or windows that fit in window size holes in buildings, or doors that go into door size spaces - if Sennheiser do not want people making something to use with their products in the aftermarket then they can start now by stopping the companies who make specialist cables, because if I decide to make accessory earpads that are superior and cheaper then I will do so and if I decide to then go on step further and manufacture them and then sell them then I will do so.