Ok. We’re talking about 2 different concepts here. It is one thing to come on a forum and start spamming and abusing the system with the signatures. But it’s a totally different story if you actually contribute to the forum but have adverts in your signature as well.
It all comes down to how it is used, and not the technique of signature link itself. To sum up, signature itself isn’t unethical. But the intention to post useless crap on the forum just to advertise the signature IS.