If you take away all the benefits of posting on a forum then no one will post on the forum. If you allow the benefits to all posters then only spammers will post. If you try to limit the benefits then if you do it too much then you lose legitimate participants while still not blocking all the spammers.
Filtering material on a forum as active as this one is only possible because SitePoint found a large number of active particiipants in the forum who voluntarily help to monitor and moderate the forum. While a number of things have been tried to automate the process none of them do any better than making it slightly easier to keep the spam out. Most of it is done by real people.
Personally, I just don’t hung up on issues like this. To me, killing and maiming is unethical; links to my site… meh, who cares? I’ve got better things to worry about.
A lot of forums—like this one—set signature links to rel="nofollow", so the issue is nullified anyhow, as spiders don’t follow these links.
I don’t mean to trivialize your question, though. It’s certainly a fair one. But I would think that, even if a forum did allow signatures to be followed, Google could hardly complain, especially if the forum content is related to your site content. Anyhow, my 2c.
All SEO is unethical or ‘blackhat’ if you’re judging that status by what Google want and don’t want. What they don’t want is people trying to manipulate their rankings.
Googlebot doesn’t follow ‘nofollowed’ links, they drop them from their link graph, as far as Google is concerned, nofollow links don’t exist. More pertinent though, you have to be logged in to see SP links and bots can’t log in so no bots can see SP sig links.
The sig links will bring you almost zero traffic anyway even where they are directly relevant. If trying to get thousands of links to your site from forum sigs is your purpose in making posts on the forum then you are wasting your time.
By joining to a forum which is related to your site, the signature link drives traffic to your site especially if you’re an active forum poster which other forum members will get interested to your site with the help of signature link.
However, since forum is updated almost everyday, chances are, the older content or older answers to some threads with signature link will driven deeper with newer threads and answers.
The tendencies here is you have to try to post new replies or create another thread whenever you can as your previous posts will get buried when other threads are created.