Two reasons:

1)
With the latest Google updates 2012 April 19th and 24th I am afraid it is simply too dangerous to link out to websites you do not know.

2)
Even after a ton of initiatives to reduce spam, it is necessary to delete and/or ban accounts almost constantly If this change can minimize the time spent on weeding out spam, that would be great as well


Important:

I have no plans removing signature links, so if you engage in discussions, you can still use your signature links to get real leads

Update:

But signature links still have to obey the general rules of course

I had to make a new change to signatures.

They are now only shown to members who are logged in.
(This means you can still have signatures and post links to get leads - but search engines won't see them)

The reason:
I got too many requests from people who wanted me to remove links - probably because they were hurt by a Google Penguin penalty.

Thanks for sharing, information is really useful, keep posting.